That is in the serial number range of the 1888 model Springfield "trapdoor." There ought to be a marking on the breechblock of US/Model/1888. I don't know what the "757" signifies, maybe a rack number at the post it was issued to. It was the last distinct official model in the series, about 65,000 made. All were manufactured from scratch, they had run out of Civil War rifle muskets to convert years before. I don't think any .45-70s were conversions, just the earliest .50-70s.
It sure looks real in your pictures although they are too dark for me to pick up much detail. Does it have a "ramrod bayonet" that slides out?
If original, correct, and authentic, value is substantial, many hundreds of bucks.
http://www.trapdoorcollector.com/
is just one of many sources.