1977 Mongoose Motomag

Cool story when I got this frame...the guy sold it to me for 30 bucks so I had to grab it! He said he's had this bike since he was 13 years old. I couldn't wait to see the serial number to determine it's age, and as you can see from the pic it's been scratched off :(
I'm not accusing him for stealing it but you never know :)
So anyways I took the bike and it came with a Tange TX-500 curved fork, which is stamped August of 1977 which led me to believe the frame is a 1977...so there you have it...cool story huh?! :D