Finnish M39, made from a Russian capture. Russians went from hex to round receivers in 1930- so anything hex with a date after that indicates it's been re-worked.
"D" chamber, 3600 is a barrel pressure check proof.
Nice find. Hold onto it unless you need to sell it.
Depending on condition (at least G-VG), value probably in the $300-$400 range, maybe a bit more.