Two versions of the TM21 were made.
The early one had a metal CRT. The later ones had a glass CRT. The attached picture is the metal one but I saw in the '66 terminal equipment catalog that they had switched over to the glass tube.
They're pretty reliable. The only thing I ever did was replace the horizontal output tubes once in the 2 years I was there.
That's the aural side of the TT25BL transmitter next to it.
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