I thought the prebuilt units were IDW-5001's and I didn't know they shipped those in place of kits at the end, interesting.
B version lighting mod got higher rated transistors in Q305 and Q306, switched from current foldback to current limited, full brightness limited to about 80%, over all power consumption dropped so it ran cooler.
I guess I'm going to have to track down a pdf of a C model schematic and compare with the original to figure this out, I know that the eprom code was changed at some point to support a newer version of the humidity sensor, I have the stuff to burn a new eprom and I have a copy of the eprom 444-0475-5, I don't know if there was anything past that.
Kevin