I am trying to find out what year my Gravely was built. The serial number plate on left hand side of the advance casting appears to be blank. I took a piece of paper and a pencil and tried to etch the number onto the paper but no numbers were visible. I looked with a flashlight and could not see any numbers. My tractor looks axactly like the one in the Gravely Convertible 7.6 Owner's Manual. It has a Gravely engine with a WICO magneto. Any help with identifying the year built would be appreciated.