The De Forest DL series first appeared in 1926. The L references the change to the UX4 base.
The DL-7 was designed as an audio output valve. This Type was made with the standard tubular envelope and also with a larger diameter bulb.
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.