So, in the 90s there looks like there were two types of ballistic vests in US service - PASGT and Ranger Body Armor. PASGT later on gets the ISAPI (in what sounds like really limited use - 4000 units). The Rangers had their own vest, which appears to have come natively with inserts in mind (first just the front, but later the back as well).
I can find out what the soft parts are made of in both examples - 13 layers of 14oz Kevlar 29 for PASGT and 30(!) layers of 850 denier Kevlar KM2 for the Rangers. I can also find the size of the plates in question in terms of area coverage, and what they're made of (alu oxide ceramic backed with fiberglass in both cases), but I can't ever find the thickness of these things. Does anyone know just how thick these early ballistic inserts were? How much was fiberglass backing and how much was actual alu oxide plating? You'd think that would be the most important dimension.
Also, I know the point of these was to resist rifle rounds - NIJ III style. What I'm not clear on, though, is if it was intended to have been rifle resistant all on its own, or if taken in conjunction with the vest, it became resistant (i.e. rounds slow through the plate and the rest is stopped by the vest). I'm assuming on its own, otherwise that fiberglass spalling catcher on the back would be kinda useless.