Not every power tube installation uses a simple push-in tube socket.
Some older ham gear, audio gear and military/avionic sockets are twist-type.
As something has to fit the index slot, a Brass Pin on the side of 811A and 572B Tubes is a mechanical index and retention pin.
These sockets where you have you insert the tube base with the pin in an L-shaped slot, Push in And then Rotate are kind of neat.
Most old tubes have a pin that sticks out about 60-70/1000th of an inch. A small group of original 811A tubes from my amps that W9KR measured showed .062” to .071”.
The pin length on new tubes may vary, as I received 572B tubes with an exposed 94/1000th of an inch of pin which is sticks out too far for one of the socket locations in my amps, as the overly long pin doesn’t quite clear other components and chassis in my amplifiers. The 572B tubes W9KR measured .091” to .094” pin extension lengths.
I have not been able to find an original specification for pin length.
When confronted with an overly long pin that hits chassis or components, a bit of Dremel tool time, relieving the excess pin length or sanding away the entire projecting pin, will sort things out.