I see a lot of questions in this forum related to furnace ages. The best book I ever purchased (7" x 7") is a book published by CarsonDunlap called the Technical Reference Guide. I paid $59.00 for the book at an inspection conference. Just turn to the manufacturers page in the book and you can immediately identify the furnace age. I don’t know for sure, but it seems like the book covers every brand of furnace ever made. A must reference book for your tool bag.