The De Forest DL series first appeared in 1926. The L references the adoption of 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 this larger diameter bulb.
Thanks to Joe Gruber for sending the image.
The envelope is 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.