The WLF finders on the RTL1000 are interchangeable with those on the Praktica VLC, with only cosmetic differences. I'm not sure about the prism. Neither will work on earlier Ihagees, as you've found.
I've read that the RTL1000 and the L- series Prakticas were a joint effort between the Ihagee and Pentacon designers. I think the RTL1000 appeared first, so really you could say that the Praktica was an Exakta copy! Actually I think the RTL is rather a good camera. You are more likely to find one with a good shutter than you are with the 'real' Exaktas.
I like my Prakticas and do find them reliable, compared to what I read. I've a few from L up to BC1,courtesy of ebay

, the prices are slowly creeping up though.The VLC's seem to command relatively high prices when compared to the RTL1000.
I tend to agree about the shutters , my very recent purchase, a VX1000 has a slightly unreliable shutter, hence the reason at looking at the RTL1000 which should have a stronger shutter.
I've just splashed out and bought an Exa 1A body with prism which I can fit the VX1000 lens and finder, got it from Germany sub £10, and the mirror is the shutter.
It will be interesting to see what its like when it arrives.
Sewin