Quite an expensive gun at 5 pounds 5 shillings (five guineas), which would have been equivalent to about $1050 in today's dollars.
That is an interesting site, but the OP has seen it since he was the OP for that thread as well.
It would be interesting if someone much more familiar than I with Martini actions could do some comparisons. I still rather doubt Stevens tooled up to make those actions for such limited sales. Of course, they might have intended to make a serious breakthrough in the British market, but I would think they would have made more if that were the case.
Jim