The older 1945-J Murray serial numbers example did not appear to have any amplifying information or details regarding a model, type or style. Later Murray bicycles would have such additional information.
I believe that in the 20th century competing manufacturers were interested in efficiency and profits, and using just-in-time methods, it may have been likely that at the moment a frame was being built, that the customer’s order and such details may have already been determined?
The alternative method might be to fabricate a big pile of bottom brackets, (or other parts), because the company will eventually use them someday, and sell them to somebody?