13 Gauge?

[J. A. EARLY]

I'm trying to figure out if back action Parker 0968 has any particular value. It is not a 12 gauge (won't fit) and not a 16 (loose). I see there were four back action grade 0 Decarbonized Steel guns made in 13 gauge. The gun is in very good shape, excellent bores, 30 1/4" barrels appear uncut, all there, checkering worn, but wood is excellent for its age. In fact the only damage appears to be freshly done with a little nick on the top of the forend where someone hit it while it was off. A little loose on face, perhaps one or two thousandths. Right hammer will not stay in the half cocked position. Otherwise all there as it should be.

I don't have a clue what it is worth, but if it is truly one of the 13 gauges, help me with any educated guess you can give me. Just one picture but the rest of the gun is reflective of the photo.

Thanks.