I agree with Mike, the rifle is about as nice as these come, though 1506 a is the nicest known by far, it appears to be fully matching and original. This is a nice rifle and very rare in this condition, - while many are known, nearly 30 recorded, the vast majority are really little more than barreled receivers, only a handful in "collectible" condition. Worse loose parts for a restoration are extremely hard to find...
Impossible to say what this rifle is actually worth, $75 is almost criminally cheap, but because I have never seen one for sale before (one with the stock and this many original parts), one would think it would be worth a considerable amount of money. Lacking a bolt really hurts value, probably more than it otherwise would on another rifle, - with the original bolt I would think $3k or better easily if listed right, lacking will hurt it a lot probably. (nice Simson 98b have done $3k and better often, this rifle is much more rare than your average Simson, - early Simson's in matching condition are rare, later blocks not so much)
Of course you have the challenge of many potential buyers not knowing the difference between a Simson 98b and an S28, some wouldn't realize any difference, - Mike's work on this subject is the first it has been put in print that was widely distributed.
Anyway, you have a very nice and rather rare rifle, - you didn't say whether there was any chance of finding the bolt, if there is even a small chance of doing so I would put some effort in doing so, I bet that bolt could, if it is the original one, make this rifle worth many hundreds of dollars more. I would offer the guy you bought it from $200 if he could find it, if he can, that rifle would probably be worth around $3k or more.