That's a good question, and I don't know the answer, but maybe these descriptions of other weapons with that mark will help someone figure it out. These are descriptions from auction catalogs posted online. Good luck!
SPRINGFIELD MODEL 1869 CONVERSION MUSKET. Cal. 50/70. SN 11451. 32-5/8 rnd bbl. Dated 1863 on beveled lockplate. Marked 1869 / eagle head over crossed arrows / U.S. on top of breechblock. There are four cartouches on left flat of stock, two ESA and two FWS. Initials W.O.G. are stamped on left side of stock near buttplate. CONDITION: Most of near-black finish on the action and rear sight remains. Lockplate has faint traces of color around hammer, but is mostly gray. Hammer has slightly more casehardening color. Right pieces have scattered spots of light surface rust. Stock has minor handling marks and mostly sharp edges. Mechanically fine. Bore is bright with some light pitting. Edwards Collection 4-58454 FS135 (2,500-4,000)
RARE WARD BURTON CARBINE. Cal. 50/70. NSN. 22 rnd bbl. Marked eagle followed by U.S. / SPRINGFIELD 1871 on left side of receiver and WARD BURTON PATENT / DEC.20.1859 FEB.21.1871 on top of bolt. Initials W.O.G. have been stamped in stock at butt and on left side at rear of receiver. Small number 43 has been stamped in the comb just ahead of buttplate. CONDITION: Metal has been cleaned to a smooth gray. Stock has two large rub marks on left cheek and another at rear of saddle ring bar, but is otherwise sound with normal dents and scratches. Mechanically good. Bore is bright but lightly pitted. Edwards Collection 4-58466 FS126 (2,000-3,000)
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