Are they new (NOS) shifters, or used?

They're not complicated--basically a pile of washers. There are no springs or anything that will go flying across the room and end up under the china cabinet that hasn't been moved for 50 years, in a pile of dust bunnies, beyond the reach of the beam of your flashlight or the reach of your yardstick.

I'd take the "bad" one apart and lay the parts out, in the order the came apart, on a paper towel, tops up. At least one of the washers is slightly domed, as I recall. I don't know if it matters if it's flipped, but it might. So I keep the tops up.

If you don't immediately see what's wrong, do the same with the "good" one, and see what's different. It takes me 10 minutes. 15 if I drop a washer on the floor and forget where it goes.