Mine's a 1954 version of the light twelve. I traded off the original round no rib bbl to a friend for one with a ribbed poly choke. Great shooter, and unlike the junk today one does not have to pack a case of spare chokes around! I sure fail to see the utility of screw in choke systems anyway. Mine does it all, from extra full to cylinder bore and with only a twist at that.
Far as slugs go, the full length bbl I have works well, but I picked up a Hastings rifled bbl some years back. I can keep every shot on a coke can at a hundred with sabots, not worth a damn with any shot version but if you desire accuracy it is without doubt the way to go.
Did own a smooth bore Browning buck bbl that shot both slug and buck quite well but a friend wanted it & I really didn't need three versions so it went.
Far as an old spare for mod goes I've seen several just stacked in a barrel in a gunshop at quite reasonable prices........I'd suggest you try frequenting some mom & pop stores and ask......those bbls are not uncommon.
As I recall the one's I saw were in Brother's Gunshop in Deland, Florida........might be worth a lookup & a phone call.