The thing with the Cadet/Cadet Major is they have an angled breech face, and the seal groove is cut concentric to the barrel, not parallel to the face.
This is why the leather seals have to be trimmed to fit and you'll never retrofit a conventional breech seal.
It is possible to fit an O-ring as a breech seal, but the bottom of the groove has to be packed out so the O-ring sits flush with the breech face, if that makes sense?
I've used PU sealant before to fill in the groove, then seat the O-ring before the sealant cures. It's more work but in fairness it makes a far better seal than the original leather one.
Cheers
Greg