The primer pump is one that typically comes from Aircraft Spruce or one of the other aviation product companies.
What is the 'locked position"? Mounted vertically, mine is up to pump and down at end of pump stroke. The reason that I want to put a shutoff valve in it is that in the event of the internal check valve failing, the carburetors can suck fuel through it and choke resulting in engine failure. I am under the impression the the primer pump is a low quality item and can fail. I just wanted to be proactive and prevent a possible problem.