Update: No luck finding a DEC-official cable. It does appear that a DEC BN25C-nn would be a plenum-safe equivalent cable.
After yet more research and putting together somewhat ambiguous statements from various source documents I concluded that the point to the SMA-906 design with the stepped ferrule is to support end-to-end extension where a sleeve over the in-line stepped-ends is used to exactly align the cable cores. There are "SMA to SMA Mating Sleeve" products (e.g.:
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=314&pn=ADASMA) that consist of a threaded shell and a center full-length sleeve for this purpose -- apparently typically used through-bulkhead to couple an internal cable to an external one.
My working hypothesis is that DEC originally specified the BN25B-xx SMA-906 ended cable for use with the LAN Bridge 100 and/or “FOCFA/FOCMA-AA RS-232 to fiber optic converter”, and then just specified it again for the VS100 to avoid a set of separate stock numbers. I guess that it came with the Delrin sleeve installed and one optionally removed it for end-to-end connections. I can imagine that one might want to extend a VS100 cable but I’ve found no evidence of a DEC stock number for an SMA-to-SMA fiber adapter; maybe they expected one to obtain one third-party in such circumstances?
It seems to be the case that the SMA-906 terminator-connector is routinely used with a SMA-905 bulkhead female receptacle by adding a short sleeve. And that there is no SMA-906 specific bulkhead female receptacle. I’ve found this description that seems to apply: “When installing SMA 906 connector into SMA 905, a
half-length alignment sleeve must be installed on the end of SMA 906 ferrule.” Thus the routine inclusion of a Delrin(?) sleeve in various photos such that the SMA-906 terminator-connector acts like a SMA-905
So I bought this
https://www.ebay.com/itm/304673572144 100/140 cable with SMA-906 ends. 77 meters is rather more than my desired length! The SMA-906 terminator-connectors fit smoothly and snugly into the VS100 bulkhead female connectors. Success. A flashlight demonstrates cable continuity in both directions. I'm not yet in a position to try putting the cable into service, but it looks very promising :->.
So if you have the same need I suggest leaving an eBay query running based on "100/140 fiber optic cable". You'll still have to look at photos to determine the connector type (vendors don't always include that information; mine certainly didn't), but that's pretty easy. Getting the right core size/design is key.
