From what I gather from reading in Sellers, that the headstamping probably came from Winchester or UMC after Sharps quit , most likely due to the shear number of cartridges Sharps offered.
It is also interesting to note that for the most part except for their long range ammo, they (Sharps) contracted most of it out to the lowest bidder..
I have several reprints of the old Sharps catalogs and they all list the 2.1 case loaded with 75 grs of powder and a 1.1 inch patched bullet and the naked grooved bullet with 70 grs of powder.
