I have read the form on this but stilll not sure. I have had my french horn since 1960 but have not used it in several years and forgot which it is. I have had it rebrassed and new case several yrs ago and someone asked me if 6 or 8D??. I do not see a serial # on 2nd valve anyplace. Help thanks

The serial number will not tell you if it is a 6D or 8D model.

Follow the leadpipe from where you put your mouthpiece in.

If the leadpipe goes forward, curves down and comes stright to your main tuning slide then you have an 8D.

If the leadpipe goes forward, curves down, and then cuts from the back side of the horn to the front side of the horn to meet the main tuning slide, then you have a 6D. You can also tell a 6D because you have two movable tuning slides for the F horn directly opposite from each other onthe back side.