ok, got it

this is not a Tesco No. 3, it looks indeed like 20 ga

I have seen this style of caliber engraving on collath guns when it had two different barrel sets - so No. 3 shotgun barrels + 24 ga rifled barrels. but it could be also the No. 3 shotgun barrels were taken of and replace by 24 ga rifled barrels maybe because the shotgun barrels were damage.
such things could happen - with snow in the barrels for example and the owner give the guns to the factory and they made him new barrels.
problem is the camber don't look like 24 ga but like 70 mm long 20 ga. I suspect the 24 ga is very undersize in this chamber?