39 vs 40 Ford Columbia speedometer adaptors???

[Kube]

Well OK then,.....February 10, 2016 to be specific.

2016? Yikes. Time does fly!

 

[glennpm]

How do I tell a '40 changer from a '39 changer from a '37-38 changer?

This is the type used for 35-36 and 39 speedometer transmissions.
As noted by Kube, the 39 is shorter.