My best guess would be that according to the contract the change was officially cut in on 26195, but there were either a. a lot of slanted antennae at the Grumman plant and they were used until exhausted, b. Grumman ran out of straight antennae at one point and utilized some leftover slanted antennae to prevent a slow down in production or c. as suggested Capt. Vick, a slanted antenna was mounted during some maintenance.