Hi,
The thing that you all think is the Fire Extingusher on the wing is a Decontamtion unit. The Fire Ex was located on the inside panel drivers side near the reinforcing gussets held on with 6 round head screws. Early Jeeps, Willys Slat Grills without glove box had the bracket on the passenger side, it was moved when the glove box was introduced. If your dads jeep has the name on the back panel, I personally wouldn't put a Jerry can on it, but off course thats up to you.
Regards,
Dave