My guess is the revolver was carried with 38 Supers because that ammo physically fit and was available. There is also a chance the revolver was actually marked 38 Largo, not 38 Super and over time the memory changed. 38 Largo in this case referring to the marking found on many Spanish copies of the S&W K frame chambered in 38 Long Colt. Then at some point the person sees a box of ammo marked 9mm Largo and it told it is similar too 38 Super. So the revolver chambered in 38 Largo is now remembered as 38 Super.