Anyone know what the 'nipples' on the lampholders are for?

Is it to reset the twist lock if the tube gets broke?

Hmm... never thought of that, might see if that might be the case....

I've always assumed those were test points as you could stick the probles of a test meter in there, they align nicely with the metal contacts for the lamp.

They are almost like push fit connections for solid core wires...

Then it's possible the same lampholders could be used for applications, such as in a recessed fitting where access to terminals from below isn't possible. That would enable connections to be made within the recessed fitting if the lampholders wewre inserted through the body of the fitting itself.

Yeah, I would go with that....