My reverse lights work intermittently. Sometimes they will switch on at the same time as putting the shifter into reverse (as expected), sometimes there's a pause before they switch on and most of the time they don't come on at all. Seems like it would not be a problem with the pin, but could be a problem in the mechanism that threads into the transmission where the bullet connectors are inserted?...

Try testing the switch with an Ohm meter, make sure it transitions from open to close all the time. You could remove it from transmission and manually activate it to test.
On my 74 this switch provides the voltage to the reverse lights.

After watching the Dr. Evil 901 rebuild video, he explains why.

There is a small detent that engages the switch to complete the circuit. That detent wears over time and when it is worn, it does not make good contact.

According to the video, you can replace them.

URO makes replacement pins for about $7 depending on where you buy. Quick and easy. Unplug boot, bullets and unscrew switch. NOTE how pin comes out. There is a ring grove on it. Compare old to new.

Thanks Mark, it's URO Parts Activating Pin 901 303 017 02 (90130301702) for use with Switch 914 613 541 02.

AA and Sierra Madre have them for less than $4:
http://www.autoatlanta.com/Porsche-Activat...0130301702.html
https://sierramadrecollection.com/Tappet-91...-76-p23683.html

Thanks, thinking it's a worn switch rather than the pin because it works some of the time. Order from Pelican has been placed for a URO switch. Will give feedback once it's installed.