Just a guess but possibly a switch that operates when the car is put in reverse actuated by the shift linkage and only functions when the ignition is on.

If I remember right it came with a push/pull switch. I believe it's pictured in the accessory book. I know I've seen a picture of it somewhere. I'm away from my picture stash for the week.

@steve-d Steve, I think in the old post on outdated system, that it wasn't possible to attach a switch to the reverse linkage.  It would have been great if you could.

@tiny Tiny I looked in the Accessories book, and it doesn't show any switch with the picture of the backup light. I think I will use a push/pull switch like you said.  If the judges don't like it then they can tell me what kind that it supposed to be.

There is a picture in this thread from the old forum that may help.

1938 Chevy Back- Up Light Switch - Vintage Chevrolet Club - Discussion Forum

@john-schildberg Take the link Minetto posted. That is the picture I was referring to. Those switches are still available today. The one below is available from Tractor Supply. It doesn't have the mounting bracket so you'll need to come up with one of those.

?c=1

@michael-falise Thank you very much Michael.   That is what I needed.  I still say there is a wealth of knowledge in those old posts.  I sure miss those days when everybody was on that VCCA site!

@tiny Thanks Tiny, I had hoped that you had one in your back pocket.  LOL