I could see another pattern pointing toward the letter 'X'...

Take the glyph as a representation of an ordered series. We have 'I' and 'C'. Its basically common knowledge that I = 1 and C = 100 in roman numerals. The 'I' is smaller than the 'C'. This relationship holds true for thier equivilant values. If we assume an ordered numerical series starting with 'I' we have:

1, 100, ?

What would be the next logical number in the series? From the two elements given, we can assume that each subsequent element is 100x its prevoius. So, the next element in the series should be 10,000. But how does one represent 10,000 in roman numerals? With an 'X', but with a horizontal bar over top of it.

This assumes that the missing element is after the 100. Perhaps it is in between:

1, ?, 100

In this case, one might assume that each subsequent element is 10x its previous. So the missing element would be 10. This, too, is represented with an 'X' in roman numerals.

But what if the missing element is at the start?

?, 1, 100

The previous two series were power series in nature. If this were to follow the same trend, the missing element would be zero. Unfortunately, I don't beleive its possible to represent zero in roman numerals, save for writing nothing at all.

Speaking of power series, these are all power series based on powers of 10.

10.

X.