I've learned that J.H. Johnston Great Western Gun Works, Philidelphia was a post Civil War (1865-1890) supplier of firearms, and like many others in the business often stamped their name on pieces they sold. Are you sure it is a Spencer? Old Reliable was a marketing gimmick usually seen on Sharps rifles, beginning maybe as early as 1876. What caliber?

If a Sharps, there are many variations. and Flayderman's 8th edition is the place to track it down. As to getting an email response from him....good luck.