Worrest had several Sngl trigger patents. Mine has two dates from 1912 stamped on the housing, Jan 2 & June 18. These would be #'s 1,013,254 & 1,029,781/82 (two successive patents issued June 18, 1912).
Walt, I had heard that Ithaca installed some of these triggers around that time frame, which led me to think mine may have been installed there. There is no engraving on the top tang of an H Lefever so no clue there.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra