It's not a Fast Deer, it looks like one of those "SnowPeaks" I think they were called. It's close, but no cigar. I think the "extra" piece you are talking about is the trigger sear, the regular old Fast Deer has a one piece steel trigger with a direct sear, yours appears to have two pieces that do pretty much the same thing, but one is the actually sear and the other is a rolled sheet steel blade. I've never had one apart, but I had one for a short while as well as a Fast Deer (that I took apart a billion times), they look almost identical, but there are minor differences....does the forend have a kinda "swoop" to it, or is it more of a gentle taper from trigger to snabble? The trigger is the other difference, as well as the little "SnowPeak" insignia that you show in your picture. I guess I am no help in "how to re-assemble", but I know what I'm looking at....hehe, sorry pal.