http://www.ebay.com/itm/220516238854?5*10 matrix. The seller completely ignored my generous offer of $12
http://www.ebay.com/itm/300621173764?Absolutely not worth the asking price of $200+ (especially in that condition), but it does appear to have tripleshot keycaps and some sort of waterproofing
http://www.ebay.com/itm/330732972326?This is a MAN's matrix keyboard. It appears to have come from one of
these and it says "new". Maybe he'll take a low offer, 'cause it's BIN for $450). This one appears to be from 1993. If this is true, then it's one of the newest ones you can get.
http://www.ebay.com/itm//300774845083?Same as the last one, but used, and comes with a case and the upper assembly (for even more switches) and some accessories. As before, BIN for $400, but waybe he'll accept offers?
http://www.ebay.com/itm/230847984550?I like to think of this as the ultimate keyboard for gaming without a mouse. It's got a QWERTY area on the left, and a ton of random stuff going on everywhere. This is probably one of the largest keyboards not counting case. Maybe it's worth $200 to someone?
http://www.ebay.com/itm/370591980448?Burroughs keyboard. I think this is the same model as the one in Toronto recently? you can actually find some information about these on GH and DT. I'm can't verify that it's got hall effect switches without a good picture of the guts or PCB.
These are always cool to look at, 'cause they are used for really varied applications. They do sometimes sell for cheap (one (Other than mine) recently went for about $25 shipped) Best of luck.
