I just found an unused bare S-100 backplane PCB marked "EXP-4 Rev. 2" and "(C) 1975, IMS Assoc. Inc.". I'm assuming that it's for the IMSAI 8080, but am not certain. I've also discovered a bare Polymorphic Systems backplane from about the same time. I'll throw in the 4 edge connectors that were taped to it. It has pads for rectifiers, caps and what looks like a regulator on it. Let me know if you're interested.