Dating a vintage Gibson L-5 (1936?)

[jpl5]

Hello. Please lend me your wisdom.
This is about Gibson L-5, which is believed to have been made in 1936. There is no label, so I don't know the serial number, but there is a FON in the f-hole. FON is X77B-2 with X77B stamp and -2 with red pencil handwriting. (X oozes and is hard to read)
I wonder if I can judge that it was made in 1936 from this B. I think the parts are also characteristic made in 1936.
The scale is 25.5 inches and parallel bracing, so I thought it might be a little strange, but I'm relieved to see the specs on the Internet.
However! This guitar does not have an adjustable truss rod.
There is a truss rod cover, but it is a dummy cover and there are no holes.
It seems that the neck contains an unadjustable iron rod. Magnets stick together from 1 to 14 frets.
Were these specifications seen in Gibson at that time?
It doesn't matter if it's a small matter. Thanking you in advance.