According to Norm Flayderman's Guide:
********************************
Gallager (
no "h" ) Carbine. Made by Richardson & Overman, Philadelphia. Total quantity of about 25,000+ (5,000 chambered for .56-62 Spencer cartridge).
50 caliber, percussion, breechloader. Single-shot, the barrel sliding forward to load. 22-1/4" round barrel.
Iron mountings, blued barrel with casehardened frame. Walnut buttstock with long iron patchbox (usually casehardened). Sling ring bar on left side opposite lock.
Serial numbered from 1 on up. Lock plate markings of two major types. Inspector markings on left side of the stock atbreech area.
The Gallager was one of the carbines which saw widespread used during the Civil War, by the Union side, and ironically had been invented by Mahlon J. Gallager, of Savannah, Georgia. The collector will observe a number of minor variations in such areas as trigger shape, sights, breech details, etc. The basic variants of the type are as follows:
Early Production Carbine; the lockplate marked: GALLAGER/PATENTED JULY 1TH 1860 (serial number).
Standard Model; the lockplate marked: MANUFACTdBY/RICHARDSON & OVERMAN/PHILADA/ (serial number);
Final Model; as above, but chambered for the Spencer 56-62 cartridge, about 5,000 produced; has cartridge extractor and a firing pin device (instead of percussion nipple). Often observed with blued frame apparently original.
********************************
It appears yours is the Standard Model. Flayderman gives no production dates at all.
Thanks for the nice photos!
Jim
An earlier thread on CWT:
https://civilwartalk.com/threads/federal-gallagher.143951/