serial number 48403-143859
Hydronic system. Never seen one until today, so it took me a bit to figure it out. Luckily the back up heat had a fried wire so it will need service anyway. It was working but not that great. Old loud and noisy.
Take your pick here;
http://www.buildingcenter.org/weatherking
Numbers can’t be right on one of these. 
Already checked, and its not there. I have the photo, so I know its right. Maybe since its water cooled it is different.
Oh well last on one the list. I will just write it up as could not determine, suspected aged.
That is what I would do.
Hydronics are problematic to date.
If it’s gas fired the AGA date can get you close but it’s best to call the mfg. if you need a good date.