Here’s some blurb I found
Surpassing the quality control requirements of the famous Telefunken ECC802S, these are extremely rare, limited production, black shield coated, avionics/aircraft grade, factory selected Telefunken G73R. Manufactured solely for Agilent Technology and Hewlett-Packard, the proprietary G73R part number and RFI/EMI black coat shielding indicate the highest quality grading on the planet. This illustrates exhaustive aftermarket testing for low noise, uniform output, low microphonics, freedom from physical defects, long heater life and an even, uniform cathode emission.
The finest West German manufactured Telefunken tubes, with smooth plates and diamond marked, these have G73R hand written on the outside black shield coating. Records indicate they were only available as spare parts for Agilent or H-P equipment. Critical use in hospital and aerospace equipment demanded tubes with extremely low noise and tightly controlled output. The ultimate tube for Hi-Fi, tube mics and mic preamps, these tubes are very fine, highly rare and in great demand. Authentic, the real deal, the bottom of the glass is diamond marked. Matched pairs and quads are available with tightly controlled transconductance from section to section and from tube to tube. Astonishingly accurate, the sound image blooms out of a dead quiet background.