Under the seat – Check the metal plate on the tractor frame, under or around the seat pan.
Side of the frame rail – Sometimes it's stamped into the side of the frame, near the front axle or engine mount area.
Behind the engine – Occasionally it’s on a tag or decal near the firewall or dash tower.
It may be a stamped number or a sticker, and on older models, it might be faded or covered in grime—so give it a good cleaning if needed. Once you find it, you can match it to year charts online or in AC garden tractor forums.