Obviously this gun has been around; and hopefully Richard can shed a bit more light. As regards your question about it's single trigger; this gun has all the later features such as large cocking hook and late style top fastener that would be consistent with Lefever guns seen with a single trigger, and the in-frame ejectors certainly look to be factory original work. Several years ago I saw a very similar AA Grade Lefever in Charlotte, NC; and as I recall it's number was also in the 17XXX range. I don't remember if that Lefever had a single trigger, but the engraving on that gun was very similar to your example.
