One other feature was whether it had just a tone control or a tone and volume control.1938 is the year that is generally believed the volume control was added.Another feature that is very good in dating is the width of the big horshoe magnet.
The guitar will probably be for sale when my friend can get an idea what a reasonable price would be for it. I'm NOT the expert that many here on the Forum truly are.
My Rick had the serial number in the end of the tuning piece; stock or whatever it's called. I'm told when the strings go thro' the actual guitar body (not a chrome endplate) it is prewar. Early models had a little octagon shaped knob for the volume control; flat in shape. Note: I have posted similar information on this Forum before.
After the war they decreased the width to about 1 inch.
Many, including Jerry Byrd (I share this belief) said the narrower magnets did not give as good a tone.
It is impossible to tell the exact year many of the early Bakelites were built because of this lack of record keeping.