Duro and El Duro are tube brands of currently unknown manufacture.

The Duro tube brand is listed on page 354 of Tyne's "Saga of the Vacuum Tube" with the remark "no information available".

Currently known are tube and boxes with the El Duro brand, and a tube marked Duro only.

A possible (however unlikely) candidate for EL DURO marked tubes and boxes could be the Duro Fittings Co. with the  brand / trade name "EL". It's unknown if they ever ventured into the tube business, and the EL name is known from 1960.

There is also a Duro Metal Products Co. (trade name Quali-Tone) who were in the radio business in the 1920s, but who are not currently known to have made or sold tubes.

Duro Electronics Co. seems unrelated as they were active after WWII.