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A certain website uses the following mapping to encode their integer ID to an alphanumeric string. Can you figure out what the mapping is?

6918        6918a
9414        9414a
13009       3010b
13628       3629b
14058       4059b
16226       6227b
18870       8871b
20740       0742c
22496       2498c
22596       2598c
27289       7291c
27487       7489c
27612       7614c
29630       9632c
29990       9992c
29992       9994c
30079       0082d
30816       0819d
30872       0875d
38289       8292d
38476       8479d
38789       8792d
39159       9162d
39180       9183d
39411       9414d
41874       1878e
41990       1994e
42065       2069e
45358       5362e
45526       5530e
45630       5634e
56297       6302f
56466       6471f
56745       6750f
59268       9273f
59566       9571f
59947       9952f
60124       0130g
60396       0402g
69993       9999g
70485       0492h
71417       1424h
72363       2370h
73651       3658h
74377       4384h
84731       4739i
85419       5427i
87422       7430i
87841       7849i
89400       9408i
89473       9481i
89669       9677i
89672       9680i
90269       0278j
90478       0487j
90663       0672j
90900       0909j
92739       2748j
107341      7351k
107387      7397k
108675      8685k
109450      9460k
110156      0167l
110681      0692l
111195      1206l
128454      8466m
128592      8604m
128596      8608m
129917      9929m
129924      9936m
130224      0237n
130415      0428n
131361      1374n
131762      1775n
131763      1776n
132642      2655n
132858      2871n
132935      2948n
133008      3021n
133013      3026n
133210      3223n
134004      4017n
134455      4468n
147332      7346o
148389      8403o
149677      9691o
149838      9852o
150209      0224p
151367      1382p
151993      2008p
152596      2611p
153389      3404p
153454      3469p
154433      4448p
154791      4806p
155017      5032p
155468      5483p
166447      6463q
167420      7436q
168449      8465q
168475      8491q
168704      8720q
169489      9505q
169519      9535q
169707      9723q
170154      0171r
170722      0739r
171246      1263r
171298      1315r
171532      1549r
171580      1597r
186195      6213s
186515      6533s
187419      7437s
198711      8730t
199001      9020t
200022      0042u
213162      3183v
213826      3847v
214049      4070v
214228      4249v
214557      4578v
214570      4591v
214971      4992v
214996      5017v
215049      5070v
215262      5283v
227550      7572w
228422      8444w
228448      8470w
229186      9208w
229455      9477w
229635      9657w
229988      0011x
230282      0305x
235719      5742x
235861      5884x
257287      7312z
257706      7731z
257918      7943z
258265      8290z
258802      8827z
259354      9379z
259524      9549z
259677      9702z
259810      9835z
260112      0138ba
260143      0169ba
261476      1502ba
267925      7951ba
268482      8508ba
269503      9529ba
271092      1119bb
271507      1534bb
271595      1622bb
271945      1972bb
278894      8921bb
279508      9535bb
280226      0254bc
280354      0382bc
280725      0753bc
289255      9283bc
289375      9403bc
289556      9584bc
289792      9820bc
290231      0260bd
290286      0315bd
298299      8328bd
298762      8791bd
298964      8993bd
298983      9012bd
299126      9155bd
299273      9302bd
299419      9448bd
299628      9657bd
300124      0154be
300720      0750be
300874      0904be
300930      0960be
301805      1835be
303221      3251be
304638      4668be
305500      5530be
305538      5568be
306079      6109be
316420      6451bf
317018      7049bf
317793      7824bf
318409      8440bf
319043      9074bf
319135      9166bf
319997      0029bg
320001      0033bg
321213      1245bg
324873      4905bg
325066      5098bg
325221      5253bg
325327      5359bg
332175      2208bh
332321      2354bh
338459      8492bh
338509      8542bh
338578      8611bh
338793      8826bh
338955      8988bh
340175      0209bi
340400      0434bi
343997      4031bi
344301      4335bi
344376      4410bi
359337      9372bj
359430      9465bj
359647      9682bj
359985      0021bk
360020      0056bk
360215      0251bk
360714      0750bk
360879      0915bk
519799      9850bz
519839      9890bz
519878      9929bz
519940      9991bz
520204      0256ca
520318      0370ca
529636      9688ca
529677      9729ca
530053      0106cb
530074      0127cb
545760      5814cc
545916      5970cc
546152      6206cc
779641      9718cz
779707      9784cz
779869      9946cz
780094      0172da
780157      0235da
780341      0419da
834479      4562df
834721      4804df
852360      2445dh
852437      2522dh
1053139     3244eb
1053189     3294eb
1053557     3662eb
1279704     9831ex
1279706     9833ex
1301084     1214fa
1301184     1314fa
1317652     7783fb
1317776     7907fb
1317960     8091fb
1468633     8779fq
1507591     7741fu
1519668     9819fv
1519688     9839fv
1519746     9897fv
1519813     9964fv
1519955     0107fw
1520134     0286fw
1529805     9957fw
1529838     9990fw
1530026     0179fx
1530037     0190fx
1530064     0217fx
1539224     9377fx
1539230     9383fx
1539274     9427fx
1539560     9713fx
1539892     0046fy
1549615     9769fy
1549769     9923fy
1549864     0019fz
1549952     0107fz
1550174     0329fz
1550386     0541fz
1559052     9207fz
1559259     9414fz
1559338     9493fz
1559851     0007ga
1560028     0184ga
1560048     0204ga
1578523     8680gb
1578536     8693gb
1578700     8857gb
1579068     9225gb
1579096     9253gb
1579117     9274gb
1579387     9544gb
1579396     9553gb
1579466     9623gb
1579846     0004gc
1579867     0025gc
1589597     9755gc
1589754     9912gc
1589849     0008gd
1589892     0051gd
1599144     9303gd
1599275     9434gd
1599322     9481gd
1599807     9966gd
1599942     0102ge
1600294     0454ge
1600312     0472ge
1609457     9617ge
1609491     9651ge
1609604     9764ge
1609646     9806ge
1609751     9911ge
1609823     9983ge
1609841     0002gf
1609868     0029gf
1618609     8770gf
1618924     9085gf
1619103     9264gf
1619650     9811gf
1620372     0534gg
1620671     0833gg
1620755     0917gg
$\endgroup$
8
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It seems that the important thing is always to

separate the last four digits of the number from the rest of it, however many digits that rest may be.

In the alphanumeric code, the number part is

the sum of the last four digits and the rest of the original number,

while the letter part shows

what the beginning part of the original number was: $a$ if there was nothing (number was four digits or less), $b,c,d,\dots,z$ if the part before the last four digits was $1,2,3,\dots,25$, then starting again the alphabet with $ba$ for $26$, $bb$ for $27$, etc.

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