After my earlier miscounting I'd better check carefully :-0 but if this is labelled B1 (and there is another one labelled B2 on *some* MicroVAX 3100 Model 20s) and it is three rows of pins ... then it is a synchronous communications connector. The same connector was used on several other synch comms options (DEMSA, DECnis, DSV11, DSB32, DMB32 and DSF32). The interface presented (X.21, V.35, RS422, RS423) was determined by the stub cable you plugged in.