Wayne, wow that is surprising that the serial numbers matched to your Pigeon Grade but with the letter M instead of P. I would believe that the M did stand for Monogram but I would think highly unusual that they would use a Monogram stock for this gun, BUT again, never say never with what can and was done at Hunter Arms.
If this were mine, I would definitely get a Research Letter, who knows what it might have recorded.
If readers will notice in your photos of the stock, there are no borders around the checkering to cover up any over-runs. Not something that was done by just any checker, but only by the best.