Not sure if I understand your question – the number on the mag plate would just be the part or assembly # for the mag. The actual serial number (if that is what you are looking for) on most of the Milburn, Continental, Comanco, etc… is (typically) found under the ridge located behind/below the tiller handle. Since there may some variations depending on when/which manufacturer owned them, I can’t speak 100% for the Budbuilt ones.