Matthew, Autocar converted 1247 M3-75 GMCs to M3A1s on Production Order T-13054. These were accepted as new M3A1s for purposes of production. New serial numbers were assigned ( 47636 thru 48882) & stamped on the frame. From talking to a few owners, the old M3-75 S/Ns that were originally stamped on the frame were X'ed out. However, the original M3-75 USA Numbers were retained & used. Unfortunately, what this means is that there is no math formula from which to derive the USA Number from a known Serial Number. Autocar kept a list of the serial numbers & corresponding USA Numbers of each unit they converted, but the one I have seen only goes up to 48088. There may be a complete list out there somewhere, maybe old Autocar files? In the meantime, hope you can find the USA Number in the places the guys suggested. If not, if the X'd out M3-75 serial number is stamped on the frame, you might try to do something to decipher the numbers under the X's. Does the dataplate ID the Manufacturer as Autocar? Could you take a photo of it next time you see the track?