Prototype set is here

[Mal Fuller]

The chassis seems to show signs of a normal life and maintenance period during your set's first years in some kind of service. Sprague and Mallory single-section electrolytics and the 1B3 date coded 1958, all suggest a period of "normal" use.
Having no apparent delay line along with a chassis area earmarked "delay" is another unsolved mystery.
I gather reading twixt the lines that this set recently came from Harry Poster. He sure has had some rare merchandise through the years.