Diffetent kind of tracking. The manufacturer retains the info on which distributor was shipped the gun. The distributor knows which gun shop they sold it to. The gun shop retains the 4473 (if that's what they called it then) and the bound book entry of the sale to the first buyer.

If the shop is still open the authorities get a warrant and go look at the docs at the shop. If the shop has closed, the owner has sent the records to BATF for physical storage and they pull them there. 

The 90 day regulation applies to the records of the NICs background check, which didn't exist in '71. Since '68, and even prior in most cases, authorities have been able to track guns to the first licensed dealer via normal industry sales records and most stores retained sales receipts even prior to the GCA.

This process allows for authorities to track particular sales related to actual crimes but avoids creating an easily searchable centralized database, which would just be a registry of honest buyers.