I believe the serial numbers are not dated and are just a I.D. number, when you mean other calibres, then yes .177, .22, .25 do pop up every now and then, .20 was never a BSA option but there was a few .20 barrels kicking about in the 80's made by certain companies, as for how much they are worth is a bit tricky as they are not overly populer with hardened shooters, as they prefer HW's but being a carbine may add a small amount but barrels are easy to get hold of and can be swapped, I'd say a minter would be in the region of £100-150.
Pete