Furnace age needed - again

Hi guys, These furnaces are kicking my A** lately. This is a Rudd. My cheat sheats tell me that the first 4 digits after the “F” in the middle of the serial # are the year and month of manuf. Well , there is no “F” in this serial #.:roll: Any help would be greatly appreciated. The home was built in 1980.

Rudd model - UGAC-125A
Serial# - CN3D103 M0480 0394

In this case it looks like the F was replaced with an M and it would be April 1980 (M0480)

Thanks Scott . It does look like original equipment.

Preston’s gives the manufacturing date as 1980 for the unit with 67.2 AFUE and 1981 for the better efficiency unit with 68.1 AFUE

Thanks Dave!