Hello,
just happened to see this post by chance.
TM stands for "tele-microphone", a designation for the early Sennheiser shotgun microphones, in the late 1950s / early 1960s. A small number of TM53 were shipped to Sennheiser, then still named "Labor Wedemark" in the late 1950s. Intended for a condenser version of the otherwise dynamic Sennheiser shotgun mics. A very small number of TM253 (with RF proof connectors) was shipped to a German broadcaster in 1963/1964.
The TM53 capsule was built for a polarization voltage of 200V, so it's not a standard KM53 capsule. Lower voltage will yield less sensitivity, thus higher equivalent self noise.
The TM53 is listed in our "Overview of microphones and capsules".
https://en-de.neumann.com/product_files/8912/downloadBest regards,
Martin Schneider