Measuring for a horn rod

[Gwill65074]

I have a '30 pickup that did not have a horn rod in it. I've been looking for how to measure it and all I find is measure from point a to point b, but no diagrams to show me what those points are. I did measure from the top of the horn shaft to the bottom, outer rim, of the cup that the piece where you put an adapter on that holds the main harness (45 3/4}but I'm not sure this is correct. Can someone help me so I don't order the wrong rod?Thanks

 

[Roxo]

This is what I found on Brattons website.
There were 5 different lengths used for the Model 'A'. To obtain the length, measure from the point where the rod meets the underside of the switch plate to the center of the keeper groove just before the bottom end. 90% of the 1928-29 cars used (5730). 90% of 1930-31 cars used (5770).

 

[Bob Bidonde]

This slide should help you determine the horn rod you need for your "A".