The seller is calling this a Infallible Single Trigger. The Infallible was invented by a Brit named Alfred Lancaster and it was installed in all sorts of guns — from Parkers to Prussian Charles Dalys. You can read more about the Infallible here.
From a rare, rare, rare A.H. Fox 20g CE with a straight grip and this great looking 20g Browning BSS Sporter to a perfect 28g Arrieta 557 and a great looking British boxlock. See all of this week's doubles now.