This switch is mostly found on Minus models, the ones that don’t have a stitch regulator, and it’s used for a single stitch.
But some of the earlier Plus models also used switches with red and black colored caps, in both the right and left handle positions (only black is available now).
How can you tell if your machine needs this “momentary” switch?
If the switch you are replacing has a METAL threaded body, it’s this switch.
If your switch body is plastic screw threads, you need the Red switch.
This is for non-stitch regulated machines that use this two-prong switch on the left side handle.