Years ago I bought a full toolbag, bag and tools looking essentially new, from some idiot who had removed it from an original '31. I don't want to know what he did with the rest of the car...
Inside with the tools was a loosely assembled pair of '30-31 license brackets, exactly as shown above but absolutely unused, paint intact in areas where it would surely have been damaged by assembly onto a plate and car.
Since then I've wondered whether the toolbag was the standard factory shipping location for license hardware that would be unneeded until the car was actually sold.
The car was quite likely from Maryland...don't know if that would have been a state that did not use a front plate in 1931.