Lack of rebuild stamps doesnt mean no rebuild, it most likely went through at least one overhaul in Europe following the war and maybe another stateside in the twentys (when they found out so many were shot due to corrosion because of improper storage prep).
These would not have a rebuild stamp.
I am not sure when they started using the rebuild stamps, any rebuilds before then wouldn't have a stamp.
As to value, the correct would come in cheaper than the mixmaster if the correct has a tomato stake for a barrel, with bad barrel you may be looking at around 500-550.
If you find a collector however who isn't concerned with the barrel condition, then you may be able to get your 650 or whatever the market will bear with the individual buyer.
BTW, a mixmaster Remington should bring in 650-700 these days, depending on your local market; for some reason these rifles vary price with geographic location, go figure
Once it has been decided to fight, one should do so to the very end, to conquer or perish...... Nothing smells better than the body of a dead enemy.
The meek shall inherit the earth - in little 3x6 plots Robert Heinlein