From the picture of the magazine tube cap and your measurement of the barrel length, it was most likely a longer gun that has been cut down in barrel length and magazine tube length. Standard length of the rifles was 24" (yours is a rifle, based on the type of fore end and butt stock and magazine attachment hardware), with a full length magazine tube. It wasn't uncommon to have them cut down to shorter lengths and/or the mag tubes shortened as well. Octagon barrels were also turned from full octagon to 1/2 round to lighten the gun or make them a little different looking. Round barrels were supposed to be standard, but the octagons were actually more commonly ordered and seen. If yours had makings on the barrel, it was originally round.
As a shooter grade gun, it should be fun. Its main collectability has been changed (lowered) because of the modifications, but its still a nice old gun with character. Do you own it, or thinking of buying it, or just trying to determine value?
