Remember, the discussion is about the 2604 preamplifier and the associated measurement microphone bodies and capsules, not the 4006 and other series mics, which are not in the same class of construction, or technical excellence.
I've used the instrumentation mics for measurement purposes, and recording, and I've used the newer recording only stuff. Both are good products, but not interchangeable, and in the case of the old instrumentation, not particularly suited for studio use.
They require very specific cabling, which uses unusual expensive connectors,very specific styles of microphone bodies, with a reasonable selection of omni-directional measurement capsules ranging from 1/8" diameter to 1". They will not work with other manufacturers products, with the exception of some rare aftermarket pieces from companies like General Radio and maybe ACO Pacific.
The recording mics mentioned are not at all related to the measurement mics. Well other than the original manufacturer. But different purpose, and different technologies.