65 satellite p/b booster

[65Bman]

Looking to replace the booster on my satellite, but the problem is that there's 2 different boosters available from almost every parts source I find. One has a rod length of 4 3/4" and the other is 5 3/8". My question is how is that measurement determined.

 

[65Bman]

Anyone?

 

[69 Runner]

I suppose it's measured from the pedal attachment point to the inside of the booster. Yours missing, or just trying to get a replacement before removing your original?