If I remember correctly, some of the fixtures were wired thru the sockets, requiring a lamp 'in place' to make the feed to the ballast.
I think you're talking about the F96, F72, and F60-T12 fixtures.
The bulbs had single pins that connected 2 contacts in the unsprung lampholders. The other end of the bulb had a spring in the lampholder.
The contacts were connected between the lines and the ballast.