I'm in the same boat as you.  I also have an old style 88 with no tag &amp; have been unable to find an answer on how to figure it out.  It seems like little to no records were kept for pre super series tractors.  You might be able to reference your engine sn with other old style 88's that still have the orig engine &amp; the sn tag.  That would only get you a range on what the sn could be though.  Good luck!!