it's the architecture of the original lisp machine from which all other lisp machines (arguably) derive, mit's CADR. there's not many of them built, since it was made by hand wiring, but there's still one or two around in a working state.
for reference here's the ai memo write up of the project http://dspace.mit.edu/handle/1721.1/5718 dated 1979, which should place it in a historic context for you
> It's the architecture of the original lisp machine from which all other lisp machines (arguably) derive, mit's CADR.
The first one would have been the CONS. As the name suggests, the CADR was the second one. :) But the CONS was a completely different architecture and the CADR was certainly more influential.
But there were a few others which might be different - like Xerox's Interlisp-D machines, BBN's Jericho Interlisp machine, Japanese attempts or some under development (like the next generation ones from Symbolics, Xerox and LMI which haven't reached the market, but were in various stages of design&completion).
for reference here's the ai memo write up of the project http://dspace.mit.edu/handle/1721.1/5718 dated 1979, which should place it in a historic context for you