It's an Imperial era Kar 98 carbine that was sporterized into being a target rifle. The sights are after market, and it looks like some additional work was done such as refinishing the receiver and (from what I can tell looking at the pics) stippling the barrel and surfaces on the top of the receiver, likely to cut down on glare when target shooting. The retaining ring for the hand guard was also removed.
As for the history, made for the German military in WW1. The carbines tended to be issued to troops that needed a handier weapon than the full sized Gew 98. Artillery, cavalry, engineers, etc. Some time after that it ended up in the US and was cut up into a sporting gun. That was pretty common back when these were war trophies in the back of people's closets or cheap surplus you could get for next to nothing at the hardware store.
Reassemble it and enjoy it for what it is. It's probably a decent target rifle if it's been cared for.
