The Sun 3/50 has 4 MB main memory onboard. Adding memory wasn't provided. This is the major design flaw of the 3/50. Third party manufacturers helped out and made expansion boards to allow 8 or 12 MB. The board on the pictures above is from Helios . A simillar board was available from Solflower.
To make the board fit into the 3/50, several chips have to be removed and put ontop the expansion board. These chips are part of the MMU and some PAL's for the memory decoding.
Important! To run the machine with 12 MB main memory you have to remove the jumper J0108 pin 1-2 on the mainboard. For 8 MB, leave it in.
The above pictures are provided by Martin Emrich.