dtingey,
Your UMC ammo is most likely pre 1910. UMC Purchased Rem. about that time then changed the headstamp to Rem-Umc. THis ammunition was loaded with small rifle primers.
JEFF WARD2,
Good luck on your search. There is some "Sticker Shock" when one sees the price of original .38-40, .44-40, etc. cartridges in original boxes. In good condition, they will bring $300 plus.
Good luck,
w30wcf