Picture thanks to Joe Torre
The 68851 is an external Memory Management Unit (MMU) designed for use with the 68k series of processors. Very few boards exist which use this MMU. Mainly due to the fact that most Amigas and accelerator cards either use a full processor which contains an internal MMU or an EC version (internal MMU is broken) which cannot use an external MMU. Plus the fact there is only a handful of applications which exist for the Amiga that take advantage of an MMU. One of the few cards that use the 68851 is the Commodore 2620.