Perhaps Grandpa's sequence is based on When writing l, M, l, M, X consider the number of I don't think I can go any further in this sequence as the next Title: Bunch of letters? It is all Greek to me


The sequence is Proof: Trivia: Therefore, the next term in the sequence is


I think, Explanation


Looks like I got stuck on the 'order within an order'; thanks OP for the hint!

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