Since I haven't gotten any replies and I found out a bit of stuff
(decstation-managers list and gatekeeper.dec.com) here is the answer to
my Q: the board in the last simms slot on the 5000/240 I have is a
PrestoServe board ("accelerates NFS speed by up to 300%"). The board
looks very much like a SIMM, except is has components on one side only,
much fewer of them and a little block on it which says "voltage
regulator" on it. 

Well, in case anybody else was wondering....

 MIKE...