I also have a 1956 310 round nose sn 6052765. Yours should be pretty close to the time mine was built pending which digit you drop from 60521660 (only 7 digits in the tractor sn). There is also an engine sn stamped vertically into the block above the starter. The engine sn will be 3 digits + letter + 5 digits, mine is 110M03357. You sometime need the sn to order engine parts as 1956 was the 1st year for the G148 engine.