I sent a query to the UK Mitsu person, asking whether their parts catalogue says enough about each part that they might be able to cross-reference the old part number with a new one. He says normally they look up a part number, and if there is a newer substitute their catalogue does tell them.
(of six part numbers I sent them, they only recognized one in their parts catalogue)
The Russian site lists part numbers but looks like many of the original part numbers (if they ones they list here match the real part numbers) have been discontinued. (ref. http://mitsubishi.epc-data.com/delica_space_gear/ )
According to that Russian site...
MB946626 from 1994-1996
MR315756 from 1996-2004
(ref. http://mitsubishi.epc-data.com/delica_s ... 83,759878/ )
That's for the PF8W
For PE8W... checking if they were originally the same compressor... yes...
http://mitsubishi.epc-data.com/delica_s ... 63,759869/
oh and they call it the "87023 COOLING UNIT,A/C" -- I believe that means compressor... though in the english manual they do call it a compressor.

The question for me now is whether the 1996 compressor would fit the 1994 one... in 1997 (e.g.) would Mitsubishi dealerships have just used leftovers of the older part, or started substituting the new part.
In a Japan auction site (like eBay but they don't use eBay much in Japan), there is an A/C compressor for a PE8W but the part number listed is different. e.g. I found this online: http://aucview.aucfan.com/yahoo/c265415509/
MB958789 -- when I plug that into the Russian site I get nothing.
I can just imagine the fun the parts dealers have.
