I was on the research staff at SRC from 1987 until 2001, and I had a Firefly as my main computer for several years until we replaced them with MIPS and Alpha based machines.
Mine originally had two MicroVAX II 2-cpu cards and a cpu+I/O card. Later the two 2-cpu cards were replaced with 2-cpu CVAX cards, but the other card remained a MicroVAX II. I think that was a pretty standard configuration. You needed the cpu+I/O card to make a usable system. The tech report on the Firefly is here: http://www.hpl.hp.com/techreports/Compaq-DEC/SRC-RR-23.html. I only did software when I was at SRC and don't have any further details about the hardware.
