30-40 Krag?

I need some info on a rifle i recently purchased, I'm pretty sure it is a 30-40 Krag that has been sporterized, it is a carbine. The only numbers i can see is a 1906 stamped near the bolt and also the number 8131 stamped about 4 times at different places on the metal, I don't think that is the serial number but would like to find out more about this rifle. I got it so cheap i'm thinking there might be something wrong with it. Thanks for your advice.