Pingree Radio Service Inc.
Pingree Radio Service Incorporated, 170 Summer Street, Boston, Massachusetts, USA
Last seen: 01-Jan-1926
In January of 1926, Pingree Radio Service Inc. placed an ad in Radio Broadcast Magazine, pg 391 (see gallery below)
It gives us a good deal of information, showing a BKNT with envelope label marked 'Pingree' in a diamond shape.
Typical voltage rating of 5v at 0.25 ampere, but a rather unimpressive plate rating of 16-90v. Surely a small OEM effort, and may not have had engraving or ink markings on the base.
I would like to hear from anyone with a specimen of this tube or box that it was sold in. Drop me a note. I suspect this was a 'blue' tube.