Find the next number in the sequence $1,7,74,\dots$
First hint:
Second hint:
To clarify: the 'hints' are really clues. They are hidden, because any of the two alone could be enough to solve the puzzle. So, anyone who is interested, can look up only one hint, or both.
There is a simple rule for both cases, all the blanks can be filled and the sequence can be continued ad infinitum.