The barrel stamps indicate C.16, and what appears to be 2 3/4 and 63.5mm. 63.5mm = 2 1/2", so I am not sure what the chamber length is.
The recoil pad installation is horrible. The original buttplate seems to be sandwiched on top of the rubber pad, but it appears as if the stock has been cut.
Anyway, 16ga, but unsure of chamber length. If it actually is 63.5mm/2 1/2", that would really reduce the appeal of the gun. 2 3/4" would be much more attractive.
Browning autos have slumped in value. I think $600 would be a stretch; doubt that it would bring anything close to that.