$6,825.00 Inland M1 Carbine

11 digit FSNs (Federal Supply Number) became 13 digit NSNs (National Supply Number or NATO Supply Number depending on which side of the pond you reside) with the insertion of the 2 digit National Codification number (NC, 00 for US origin) between the Federal Supply Classification (first 4 digits, FSC) and the National Item Identification Number (last 7 digits, NIN or NIIN).

So a FSN 1005-621-2603 becomes NSN 1005-00-621-2603. Checking that NSN it is still a valid number in the NSN database:
MAGAZINE,CARTRIDGE
Cage 19204 ARMY, UNITED STATES DEPARTMENT OF THE
STEEL,BLUED,3.937 IN. O/A LG,1.915 IN. W,0.750 IN. THK,15CARTRIDGE CAPACITY,CAL..30 CARBINE

Searching the NSN database, I can find the FSN/NSN for the 30 round magazine - 1005-716-2176/1005-00-716-2176:
MAGAZINE,CARTRIDGE
Cage 19204 ARMY, UNITED STATES DEPARTMENT OF THE
STEEL,BLACK OXIDE COATED,5.630 IN. LG,1.915 IN. W,0.753 IN. THK,30CARTRIDGE CAPACITY,CAL..30 CARBINE

The NSN 1005-00-521-2441 does not exist in the NSN database and I can't find it in the cancelled NIN list either (there are other 1005-00-521-xxx numbers, but not that one). There is a 521-2441 NIN with a different FSC, but that is a cabin temperature gauge for a Boeing 757. I'm not finding any other magazine NSNs using reverse description searches or likely mistypes or partial matches of the number on the label...

So my guess would be its a made up number.

NewScotlander wrote: I know this may be hard to believe but I have one in nicer shape. Not for sale though. 3094590 https://photos.app.goo.gl/mZ1MYq8MbEDL8Qfy5