Unique plumbing issue in a 300-400 year old house

[cloudsix]

Good observation. And no, indeed unlikely. But it will stop you being able to carry out an air tightness test on the remainder of the system which would seem to have been what your plumbers should have done initially instead of changing traps and fitting valves at random.

If it passes a tightness test, then we can assume a trap. If it fails, you may just be able to find the leak with gas leak spray or similar. If it won't take a pressure at all, then there may be a 20mm hole drilled into the pipework - and yes, I've come across one!

I agree it's definitely worth sealing, either way.

Tightness test sounds like a good starting point once I'm sure this shower trap is fitted properly. Just need to find one with a seal the correct size.