Why two different and incompatible options?
Or, more generally, why can’t I ever read some USB spec and get out with less questions than I had in the beginning?
These are for niche needs. For most applications neither is necessary. If needed and space is constrained the single pin variant allows additional connectors to be packed together on a single PCB. The dual pin option doesn’t, it takes up space that could be used for additional connectors.
But why not make a dual screw design by adding one more screw to the bottom of the current single screw one? Would be more compact and allow them to be more cross compatible.
PCB are designed with things on top of them. Typically they are mounted with the bottom of the PCB at the bottom of a case. So following normal conventions there wouldn’t be a place for the bottom screw to go into something. Unless you used a bespoke case. It’s much easier to have the two screw design place the screws above the PCB.
It may seem like a minor change, but it costs substantial more in design time and effort. New folded steel cases and injection moulded cases would have to be developed. Designers would use a different port, or worse deploy their own two horizontal screw design. There would then be several two screw designs (metric/imperial/very close/very far etc).
Anyone with the niche need of the two screw vertical design would likely rotate the whole connector or use a flex cable to join the connector to the main board.
I se no advantage at all for the dual pin design.
It looks exactly like what I imagined by screw-lock USB connector, but the single pin seems to be a really inspired design somebody had and made the entire committee angry for some reason.
The single pin only resists force. Two pins resist torques being applied to the usb connector. The single pin does resist torque but it uses the connector potentially damaging the PCB.
