The photo from didjamatic appears to depicts a slider over rubber dome.
It appears that 631x and 651x were parallel series, as I've found examples of both each in both the "angular" and "curved" style cases.
HaaTa's old-style 6511 with the same sliders as didjmatic's:
http://geekhack.org/index.php?topic=12244.0A 6512 with 631x-style backplate and additional keys:
http://www.digicoll.com/tecladosyratones.htmlMost (-TA and -VA) have three extra keys in the navigation cluster, but there are both angular and curved style ones that don't.
I'm guessing that depending on how much you paid, you could get rubber dome over membrane (651x) or "mechanical" over membrane (631x).
Also, Acer even used that sleek angular case (and I do love Acer's angular cases) for the 6011 mechanical:
http://geekhack.org/index.php?topic=29130.0Pictures of an angular 6011 with PCB shots but not switch shots:
http://geekhack.org/index.php?topic=10095.0("11" is ANSI and Asian, "12" is ISO, by the way)