As a matter of interest, there is a CORRECT way to fit the cup discharger and that's why they were fitted to selected rifles - not necessarily clapped or worn out rifles either. In short, the cup is fitted and then the internal nozzle is screwed in or out so that the end of the nozzle is seated FIRMLY onto the end of the barrel as opposed to the rear end of the cup base being sat onto the nose cap. This way, the load of discharge is taken squarely on the barrel and therefore down through the body and into the butt. That is opposed to being taken wholly onto the nosecap screws and down into the fore-end.
A few years ago I detailed the full instructions that I'd taken from the 20's training notes and it made a few owners sit up and take note. As I recall, many didn't even know that the internal nozzle was screw-in and out-able. It is indeed and was a replaceable part. I think it was an old BSP thread too. BSP eh!............. I bet that's got a few old 40's engineering apprentices wracking their brains