I've found that the 37 pin connector on the back
of the VAXServer 3100 is a
 DECNet connector. Depending on the pigtail it connects to either a V35 plug
 (big square thing) or a DB25 that connected to a DEC synchronous modem. 
It sounds like a synchronous serial adapter that can be used for DECnet, not
a DECnet-only adapter.
--
Eric Dittman
dittman(a)dittman.net
Check out the DEC Ethusiasts Club at 
http://www.dittman.net/