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.