Spec
3940
5136
Thickness
9
10
Diameter
36
37
Caseback
snap/display
screw/display
bracelet
leather
WG Bracelet
Some other thoughts, the 5136 seems to have a bigger font on the "day of the week" subdial. Also the 5136 is marked for 25M WR, whereas the 3940 is marked WR, but with no depth.
Hope this is helpful
A