The issue is you are using the modern word 'pair' for a medieval word. Pair developed from Latin, paria or par which means equal or similar. Nothing about there only being two items that are used together.
Ahh, stupidly I didn't think to look at the etymology of 'pair'.
So, after a bit of digging, in Middle English (via Old French), 'pair' means any number of alike or equal things as you say (e.g. Chaucer using 'a peire of beads' to mean a set of beads).
However, as it applies to any number of multiples, it was used to refer to what we think of as pairs as well (that led me to the term 'brigander' which I'd not come across before..seems to be an early form of brigandine that did have a front and a back half).
Chaucer uses it in both senses in the Canterbury Tales.
"Som wol haue a peire plates large" (Knights Tale)
"And on hir feet a peire of spores sharpe" (Prologue)
In the Memorials of the Order of the Garter, you get this entry for 1354
Now, is that referring to two seperate 'coats of plates', or to two sets of plates, a front and a back set?
Interesting. Cheers Randall. Sorry for drifting off thread a bit everyone.