I may have to look through some of my tech service manuals for specifics, but...
The Apple High Performance Processor module is exactly what the name implies: a high-performance processor module. It comes equipped with a PowerPC 604ev (or 604ev Mach 5), running at a minimum of 250MHz, with a max of 350MHz in the last versions. These boards also feature integrated high-speed caches, from 512k up to 1MB in the 300/350MHz variants.
These boards were virtually identical to one another - clock frequencies were set by setting the resistor matrix on the back of the card that you noted previously (a single resistor selects the speed - if you have a resistor soldered in the 250MHz spot, for example, your card runs a 250MHz processor). Though 400MHz was an option, Apple didn't ship any cards that ran at that speed, probably because they didn't want to compete with the PPC 750 in their newest machines.
These cards definitely work in the 86/9600-series machines (these boxes shipped with them), and should work in older models such as the 85/9500s with a similar 50MHz system bus (no guarantees, though).
