The rep is L serial and the gen is M. I don't know if that makes a difference to the font used, but the caseback has brushed SS on the gen and polished SS on the rep too.
I had a 312 rep for a while - I sold it when I bought a Gen 329. Side by side they were almost indistinguishable (apart from the GMT of course). The lume is great. The real tell is the movement, but that's not visible when it's on your wrist. The AR is good, (the lower quality sapphire is where it falls down) but I think it's a great rep.
I have a gen 329 and a rep 312. Side by side, they are almost identical (GMT complication apart). The AR is pretty much the same - it's the clarity/quality of the sapphire on the gen that makes all the difference.
Have a look at the teardown review Angus did: http://www.rwg.cc/topic/138469-the-ultimate-pam111m-teardown-review-150-images/
It show's you most of, if not all of the differences between the H and N models, some of which can be assessed without pulling the watch apart.
The black ringed subdials and brushed lugs is what swung it for me. The clasp on the 16520 is a bit non-premium compared to the 116520, but overall, I prefer the more vintage/tool-watch look and feel of the 16520.
My gen 1680 with 93150 has 580 endlinks.
One thing you do need to do when modding MBW cases is reduce the space between the crown and the body of the watch. Even with a gen crown and tube, that little 0.5mm step of metal is a big tell. The dials are great though - almost indistinguishable from the early T < 25 replacement dials (without a loupe of course).
I managed to get hold of a gen 93150 from 1978 with 580 end pieces and put it on my MBW Great White 1665. I don't know if it's psychological, but it makes the whole watch feel more gen.