3A is not a D8 serial number but 13A is but there is too many figures there as not that many were made.Look on the left rear side of the block below the head and there should be a plate there with the SN on it or on the left rear of the housing above the drawbar near the top and see if you can find the number there and if the plate is missing either place the factory always stamped it in the casting so you may have to scrape a little paint off to see it between where a couple rivets were driven in to hold the plate.It looks like it could be an old 2U or the 13A serial number.I can,t see it too well but the hood on tractors up and including the 13A has a straight hood all the way back to the dash where the 14A and up tapers back at the dash which is narrower.
Even with the curved exhaust I think I'd still put a 5 gallon bucket over the stack to keep the rain out.
Those are Bosch magneto model numbers for the 2U & 13A tractors.
1946 - 1953