Poyer reports Remington made m1903 barrels until Nov. 1942, and the 1903a3 barrels lack the extra machining needed to fit the sight collar for the m1903...as such any 1903a3 barrels produced in droves after production changed 11-42 ish would not be compatible. Since we know that the transition was not instantaneous or smooth - as evidenced by my receiver and others from {11,12}-1942 clearly starting as A3 but finished as m1903, I had some hopes.
However, I think those hopes are already dashed by the fact that this barrel is clearly short chambered. Simplest explanation is it was in a bin of lat m1903 receivers that were never completed in the changeover, and was bought much much and fitted with a NOS replacement barrel.
I wouldn't say "surprising" - I imagine if it was forward deployed and serviced with a new barrel, they would have put whatever replacement parts on it they had.
Guessing is part of the fun!