I have the original owners manual in my March '66 Dearborn coupe as well as a repro copy. The repro is an amazingly accurate copy of the original. On the back cover both are Form 7833-66. The March original is Second Printing, January, 1966 (inside front cover). The repro says Third Printing, April, 1966. On page 40 in a section titled 'Circuit Breakers' (??) both have the same identical picture of the fuse block with the lower fuse labeled as "20 AMP FUSE LIGHTER AND EMERGENCY FLASHER".
By at least January someone had decided that Mustang fuse should be a 20A. Who knows if it ever actually became a 20A. FWIW the underdash harness in my July San Jose car has the same fuse block as the March Dearborn both with the small slot labeled as 20A. The July main harness (which includes the fuse block) has a Ford label of C6ZB-14401-F.