Unspellable is most correct.

Why do you need to know?

Because it is part of being a double rifle owner! I need to know as much as I can garner about old British Double Rifles.
It is not esoteric, nor is it benign. People can make a disastrous decision based on a correlation between fact that cannot be made. That results could at best damage a fine old double or worst damage the owner.

Just because you don't need to know doesn't keep you from potential harm.

The double is a differant animal. While you are waiting for traditional bolt rifle pressure signs to appear, you can already be damaging your double taking unneeded risk.
Just my opine here.

I suggest you go do what you need to do and be happy!