Hi,
I once owned a Lancaster Oval Bore 500BPE D/R and I believe they were meant to be paper patched. It is a beautiful rifle, and one of many that I shouldn't have sold, but such is life. I think that mould is a solid base ant that is not a hole for a plug but the sprue hole. The sprue would be cut probably by the shanks of the mould handle. Being a M/L i would think that the patch would be fitted as you loaded, but would need to be pretty tight.
Just my thoughts.
LesLeeSpeed