Conn french horn serial location

I just picked up a Conn 6D french horn last night from a friend of mine. I'm borrowing it to see if it's feasible to get my chops back quickly, and if I can I'm picking up my old horn (Holton Farkas 179) from another friend later this month to start playing again.

I'm curious about this Conn 6D as it's a beat up school horn that I can't tell how old it is. I found the 6D marking on the mouthpiece receiver, but i can't find a serial ANYWHERE...where is it?

many thanks if you can help.