Most likely not a pigeon gun as English pigeon guns usually weighed slightly below the 8 lb. maximum allowed weight & were proofed for an 1 1/4 oz load.

Maybe a gun set up for driven red grouse by someone who preferred a pigeon style rib. I've read that the first shot at driven red grouse is normally taken well in front of the shooting butts.

Hopefully one of our UK contributors will comment.