Form 1508 is a "Statement", Statement of Shipper of Firearms
On the form just under the "Statement of Shipper of Firearms" Domestic Mail Manual C024, retain form for 1 year.
Therefor the understanding is that a Shipper of Firearms must file with the PO a "Statement" (form 1508) and that this form/statement is to be retained for 1 year.
Now would the FFL need to file one again next year?
I have personally talked w/ a couple of Post Masters about the form 1508 and they do file it, some in the cabinet and some in the round.
IMO; If one reads what the ffl/dealer is making a Statement about on form 1508, one would know if they are presenting parcels that are customary trade shipments addressed to bona fide recipients, ie; ffl to ffl.
**Info from the USPS below, fyi**
1.5Manufacturers and Dealers
Handguns may also be mailed between licensed manufacturers of firearms and licensed dealers of firearms in customary trade shipments, or for repairing or replacing parts.
1.6Certificate of Manufacturers and Dealers
A licensed manufacturer or dealer need not file the affidavit under 1.4, but must file with the postmaster a statement on Form 1508 signed by the mailer that he or she is a licensed manufacturer or dealer of firearms, that the parcels containing handguns (or major component parts thereof) are customary trade shipments or contain such articles for repairing or replacing parts, and that to the best of his or her knowledge or belief the addressees are licensed manufacturers or dealers of firearms.