Are these boxes common?
Milwaukee Ordnance Plant manufactured 235 million rounds of Cal .50 (not 12.7x99mm). I don’t know how many were AP. Probably a bunch.
not at all common to me, as a box. 42 was, i believe, 1st year of production.
The majority of FACTORY .50 AP ammo encountered is of WW2 vintage, with DM, SL, TW being the most commonly encountered. The other plants command a little more of a premium, but not much. M and UT (U), for example, are less often encountered than even FA.
A sealed box would be considerably more valuable than one that has been opened and the condition of this particular box isn’t the best, but is still a better find than most.
If a normal WW2 AP round is a $3 or $5 item, the M might be $7-$10. But you’d find pricing all over the map if you looked. There’s a guy selling a pulled .50 AP Bullet for $99.