Age of unit

Can anyone help me with the age of this Rheem AC unit, Mod. RACC-042-JAS, Serial 3936FO7919211. My Carrier Blue Book and Prestons Guide hasn’t helped much. THANKS!!! In advance.

Usually the four digits following the “F” in the serial number represent the week and year of manufacture.

In this case, the seventh week of 1991.

I get 7th week of 1991.

Thanks! Larry & Bill