I never feel good when disagreeing with experts like T-Mar, but I think it's a 310, based on the steepness of the head tube.

In either case, you got a quality steel frame bike which you are already riding for $40. That you got good deal goes without saying in either case.

If you are new to cycling, I urge you to get someone to apply fresh lube to both wheels and the bottom bracket, especially if the bike has been sitting unridden for a while.