I tend to agree. However, I have seen numerous photos proudly posted online of firearms produced post GCA 68 that do not conform. I wonder if BATFE makes note of these guns and what ramifications this/these may involve.
I do know if you have a machine shop and purchase a 80% frame (let's say a 1911), if no serial number on a frame as a finished frame it is legal to own but not to sell.
In order to sell it I believe one must have a Title II permit/license and declare the serial number with BATFE.
How far off is my knowledge?