'shooter88' - The U.S. Armory at Springfield, Mass., did not put dates on the Trap-door Lock-Plates, except "1873", on #1 thru #80,000. Later, the lock-plates were stamped with just an Eagle and Springfield.
The breech-blocks are dated either Model 1873 (sometimes appears like '1878' due to poor Die strike) or Model 1884.
IIRC - 'Dated Lock-Plates', (other than legitimate 1873 ones), were made by surplus dealers (like Bannerman's) putting together parts guns in the 1920s & 30s.
You should be able to find a very fine, 'shootable', model 1873 Springfield rifle for well under $1,000 in the U.S.
IMHO - You should run away from this one! Buy a cheap Joe Poyer/Craig Reisch Book: "The .45-70 Springfield", by North Cape Publications, and educate yourself.
Save your money and shop informed. Springfield trap-door rifles are a bit of a 'sleeper' and very good ones can be found in the $700-800 range. Search GunBroker to get an idea of what they sell for.
Al Frasca (and his books) is a great authority on 'trapdoors. The U.S. Springfield Trapdoor Rifle Information Center