There are no precise dates of transition and quantities available. The closest I can come to the truth:
After Neumann developed the K8x Mylar capsule family, the final few KM(2)54 were delivered with the new capsule. To link a capacitor capsule which is grounded on one side, these mics received a coupler ring between the amp body and the capsule head, to accommodate the circuitry of the KM54. This coupler was also retrofitted by the Neumann service center in Berlin to any earlier KM54 whose nickel capsule failed, and which were fitted with the new KK84.
In my chronology based on data I have collected over the years, only a very few KM54 were made after January 1964. These received the then new KK64/74/84 (Mylar capsule plus coupler ring).
Put simply: any of the few KM54 issued after 1964 would have the Mylar KM8x capsule system and the coupler ring.
