From what I remember, the Nikon 10 pin adapter is their own proprietary serial implementation and dates back maybe to the N90? They actually pretty freely shared it with Kodak to build the DCS 200 and 400 series digital backs, which use that port to sync the digital functions of the back with the camera body. The DCS 600 and 700 series are based on the F5 body and I think use the protocol internally(leaving the port on the front free). I've been inside my DCS 760, though, and it very much looks like a "Digital back for an F5" just without the obvious place to detach it. The DCS 14/n and SLR/n have this same sort of feel also, although use a lower end body.
The original MC-34 and MC-35 cables for use with Photosecretary used the then common peripheral ports-DB9 RS-232 for the PC version and Mini-Din-9 RS-432 for Mac. BTW, I have the Mac version of the cable(along with the Meta-35 set-up) but have never found a copy of the Mac version of the software. If anyone has a lead on any version of it, please let me know!
Unfortunately too, for those of us who use Macs, we've been left behind by Meta-35 also on our current computers since AFAIK they never updated to 64 bit. The only version I have runs on I think OS X 10.9(Mavericks) or so to macOS 10.14(Mojave). I wrote to them at one point and asked for an older version of the software that would run on 10.6.8, since that's what my scanning workstation runs(compatibility with NikonScan...) but I don't think the version they sent me worked...