There are a gazillion firearms for sale out there today that have no serial numbers, and they're all perfectly legal to sell/trade/buy.
Firearms manufactured before a certain date (circa 1940's I believe) were not required to have serial numbers, therefore many manufacturers didn't bother.
I have a Mossberg model 144 .22Lr that has no serial number, and it never has.
Yeap, you are correct. The 68 GCA changed that, If you built that firearm before 1968 you would be required to serialize it prior to sale or transfer. If it's for your personal use it is not required. Atleast thats my understanding.