So I was working on a Display Stage Lighting 6" Fresnel today. (Amongst a few other fixtures). Curious fixture in that instead of the standard medium prefocus (P-28s) socket to it, it’s a totally round shaped fixture using a 400w G-30 incandescent lamp and has no flair out at the bottom of the Fresnel to make up for the extra height of a normal Fresnel’s lamp in seat height. Curious and given Kliegl, L&K plus perhaps Display Lighting all had the Fresnel fixture on the market in the 1930's as with it the first (perhaps pre-modern how it’s done or normal) introduction of the Fresnel or Leko’s in history, and it’s unknown to me when exactly is the date of the introduction of the pre-focus lamp socket but in theory the late 1950's, I’m thinking with some research the forum can help me identify the time line for this fixture further. It’s using hex nuts and beyond that, a sort of top lock hex nut on the yoke. What year about did the hex nut between say the 1930's and late 1950's did the hex nut come into general use in industry? This given especially given this fixture is still using round head slotted screws, one might further refine it’s timeline for manufacture.