The CV1670 is an indirectly heated triode. The prototype was the 4D1. The old GPO code is POVT100B. The STC commercial code is 3A/172B and this exhibit carries both the commercial and the CV Type designations.
The STC logo and commercial Type designation.
The B7 base pins.
The pinch on the left, the heater passes through the cathode tube six times. The control grid can just be seen to the right of the lower mica.
Thanks to Mark Mallison for sending the images.
The classic envelope is mm in diameter and, excluding the B7 base pins, is mm tall.