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.
There are some omissions and errors in the TRG, but it’s the best I’ve found. And when one combines it with Prestons Guide, one almost always has one’s furnaces covered.