The De Forest DL (the L for the UX4 base) series first appeared in 1926.
The DL-15 was designed as a sensitive detector. Bill Condon tested several DL-15s to check the De Forest claim that the DL-15 gave superior results with weak signals. The above article gives details.
Thanks to Joe Gruber for sending the image.
The envelope is 32 mm in diameter and, excluding the UX4 base pins, is mm tall.
Reference: Bill Condon & Joe Gruber.
Absolute Maximum Operating Conditions¶
Updated September 22, 2020.