I know Tucson AZ was using these type of lights in the 50's, but is there other states that use these lights in the 50's?
Tulsa has used yellow signals since the late '40s or so. The city began using backplates in the mid-'50s. These were typically made in-house out of a thick rubber-like material, sometimes with clipped corners, rather than radiused.
The city switched to a somewhat thinner material in the mid-'70s, finally stating to use manufacurer-supplied backplates in the late '80s.
There are still some signals around town with the home-made backplates, usually on the few remaining city-fabricated truss-arm installations.
SM-T580
We already had a topic on this...that you started:
https://www.aaroads.com/forum/index.php?topic=21935.0
Is Witcha Falls TX is the only place that used Yellow Econolite Bull's-eye Signals with Black Backplates in the 1950's and 60's?