Radcliffe's are highly reputable gun-smiths. I'm perfectly certain that this wasn't an anomaly unique to the Bamber gun. Many would have been sold, so I imagine it's idiosyncrasies would have been reported.
Yes, reported to those in the know, but Sheila wasn't to know and obviously neither was Jeremy as he hasn't even mentioned anything about its workings, not even to bolster his own situation.
Come to think, EP can't know a lot either.