I just bought a 1936-37 Supro Electric Lap Steel. The serial number (B600) suggests it's from 1936.
The case, and maybe guitar itself, was manufactured for Gravois Music Shop, where the legendary Mel Bay used to give guitar lessons in the mid-'30s.
From all the research I've done, this is the oldest one I've seen so far.
A little background info: (src: )
Supro debuted in late 1935 when Dobro and National merged, and began selling in catalogs in March of 1936, suggesting that this was one of the first Supros ever built. This explains why I can't find much information on it!
In 1936, after moving from L.A. to Chicago, stopped making them of metal and switched to wood. The single treble-side knob indicates that this is a 1936 model, but it lacks the pick guard, suggesting a 1937.
Any info would be greatly appreciated! Thanks!