Ahh yes Benny, infact...
There are several differences in the sole plate, the 612 has 2 metal plates on where the opening to the brushroll is, where as the 616 doesn't, and as you said, the anti lean forward thing near the rear wheels, also, the 616 has a strip in the centre of the sole plate, I guess for a safety thing, possibly.
The 616 also doesn't have a handle on the bag door, which I believe only the 610 and 612 had, to assist with opening the bag door, but the 614, 615 and 616 never had it for whatever the reason.
I meant to mention, my 612 leans forward slightly and when you push forward on it, there is no real feeling of it stopping from going any further forward, there in no, "bash" feeling when you stand it upright and lean forward on the handle slightly...where as, when I stand the 616 upright, you can feel a big "bash" as it hits and obstacle to stop it from pushing any further forward, so it will stay at a 90 degree angle.
I hope that makes sense, because I can't think of any other way to explain it, lol.