The numbers can match (receiver and receiverl extension),,but I was talking about a change of barrels involving only the tube itself.
You use the guns original receiver extension, mag tube and forend wood.
Putting a different bbl into the original receiver extension still gives you matching numbers,,but not the barrel it left the factory with.
It takes a couple of special tools to do but most shotgun bbl gunsmith types should have them yet.
M12 and M97 use the same tool & bushings. Same tool with mag tube and chamber bushings sized for 410, and you can change bbls on the M42 too.
It's just worth checking to see if the solid rib bbl itself was available on the (field?) grade of gun you're looking at for the year it was made.
Perhaps it was, then at least you'll know you're dealing with a factory configuration. It still doesn't tell you about originality though.
When a large quantity of M12 original 20" riot bbls were dumped on the market years back,,there was a small industry to take near new condition field guns and replace the 28" & 30" with the 20" bbl.
Presto,,high dollar riot guns. Why some even appeared w/ PD, Military and other security type markings.
Always worth checking out to see if the gun configuration was even available in the yr/mfg.
Many of those riots were done on guns too early for the bbl address type that was on those surplus bbls.
It pays to do some looking.
Hope it turns out to be a 'real' one. They're beautiful shotguns especially in 20ga. The solid rib is a favorite of many over the vent. Nice classic look.