So what is the issue?
There were two different brackets used on the "standard" Model A narrow bed. The wide bed exclusively used the latter of these brackets.
I say two brackets: those adapted to use with a "drum" taillight, and those adapted for use on the "teacup" taillight. The difference is in the "arm." Drum brackets are "straight" between the two bolts attaching to the light and teacup brackets are "C" shaped (to give room for the wiring on the back of the teacup.)
Both are reproduced and available from parts dealers. Both rely on a wood block to attach properly to the bed - details of the wood block vary depending on the assembly plant practice at the particular time but is thought to have been made from material left over after cutting the square "cutout" (frame hump) in the bed. Ford didn't waste anything.
This like in many things Model A "commercial" is subject to variation. The use of a station wagon type bracket has been documented on early narrow bed trucks, but few examples as provenance.
Or perhaps your question has to do with "augmenting" the relatively poor single stop-light with bed-pocket mounted light assemblies? This like a perfect use for teacup repops with LED.
Joe K