I don't quite follow Larry's inferences. There are four pairs of letters given, as well as the first letter of a fifth pair. Four other letters (represented by "x") separate the pairs from each other.
I said the sequence was not mathematical. Specific subject area is 20th century US history.
And from this point, every 4 hours (or there abouts) I will fill in one of the "x" fillers until someone sees the pattern.
Clearly not a good puzzle in original form. Maybe I should have given more of the letters in between to start with.