#77 or #78 what's the difference?

[btmccutcheon]

Looking at GEC's #77 Yankee Jack and the #78 American Jack, I'm having trouble telling the difference other than maybe a 1/16" difference in length and cover options. I haven't had my hands on either of them so is there really a difference?

 

[Rockon75]

Pen blade location. Spring thickness (wider stiffer on the 78). Just off the top of my head.