What's the trick? (headlight switch madness)

[Screws]

Hi Folks,

Just reassembled my '65 after a full rewire, and replaced the headlight switch. I found the switch in one place and the knob and stem (1 piece) in another- both apparently period correct new. The two pieces are not marrying up- the stem fits (triangle stem-triangle hole) but it doesn't lock in. I don't want to force it.... and the button on the side of the switch that seems to have something to do with it isn't releasing or catching anything (the stem remains not locked in). I'm throwing my hands up and asking for some wisdom on this one.

Thanks in advance. The fact that this forum exists is very cool, and appreciated.

-Screws
Baltimore, MD