Hello mdh; there is a lot of good information on the TRS-80 Model 1 and the EI on this site. As to the various revisions to the EI and the Keyboard unit, I believe there were four...
1) Original with no mods (connected with ribbon cable)
2) Powered buffer box between the two units (used 74LS244's to buffer the signals PLUS a modified ORANGE power wire in the EI)
3) DIN cable mod (route memory RAS, CAS and MUX over DIN cable PLUS a modified ORANGE power wire in the EI for the older buffer box)
4) New re-design of the EI that generates RAS, CAS and MUX on the EI itself. EI board looks different with memory relocated from original.
Care must be given to the ORANGE power wire as Radio Shack used a modified GND trace in the EI to route the new power to the buffer box.
The DIN cable modification is described here..."TRS-80 Model 1 Expansion Interface Buffer Mod.pdf" (
http://vintagecomputer.ca/files/Radio Shack/TRS-80 Model 1 Expansion Interface Buffer Mod.pdf)
Buffer MOD
Photo's of the EI modification and discussion is here..."TRS-80 Model I Expansion Interface" (
http://knut.one/m1ei.htm)
As to the expansion interface power supply TRS Ian has one on Ebay (shipped from Australia) or you can build your own. I posted info for doing this on-site if you search for..."Expansion Interface Power Supply" 2/27/19 by Dawsoca.
Expansion Pwr Supply
TRS Ian's unit works best when using 240VAC with the monitor on top of the EI. Wavy lines in the display are visible. In the US, the 120VAC units do not seem to have this problem. Mine works fine and powers the keyboard unit too.
>>> Charles