The FIRST year production of the 44US used a different receiver sight (Lyman 57S, if memory serves) and was not drilled and tapped. Those were made in 1943. Following years, about 4.000 were D&T'd for a scope mount. Change to rifling pattern may have taken place at same time. Word was the Mossberg would lead lap their bores- after rifling, a lead slug coated with fine abrasive would be worked thru the barrel, lapping the sharp edges, and getting excellent accuracy as a result. If you want to PM me with SN's, I may be able to give you year of manufacture.
BTW, you would not BELIEVE the price for some of the older Mossberg peep sights. Shortest verse in Bible is John 11:35. "Jesus wept."