WE PDW Versions
 

I'm picking up a WE PDW GBBR from a seller here, and I had one before, but had to let it go due to financial constraints. I've noticed everywhere that retailers have "new version" marked for theirs, and I've read that there are three versions.

The one I used to have used a bolt reciprocating on a spring/guide rod system, and I know there is a version using a return spring in a rubber banding type system; what is the third type? Anyone know if there's a way to tell the difference externally?