After you have handled or been around in the mid-five-figures worth of vintage radios, you will learn not to reason from the specific to the general.

Magic words here a

1920s
Home Brew
Single tube

a) Suspend all other expectations, and deal with what is in front of you.
O1A tubes come in several shapes, from globe to ST. I have seen one (1) "G" type as well, so anything is possible. There are also several other candidates that would do, including an OO, 30 and 112 amongst others. Some would require a different filament voltage. But, the O1A is/was the go-to as they were very common, and much cheaper than the alternatives.

b) Variable capacitors are/were expensive relative to fixed caps. So, your home-brew hobbyist likely picked a fixed cap at some value between that specified for a variable cap.

c) Fixed caps are/were expensive relative to nothing - so your home-brew hobbyist 'went without'. He/She (a great many of the earliest hobbyist were girls/women - why? Most of the parts were made by women). He/She was probably listening to a nearby "torch" station with little or no competition, so targeting to a specific frequency would be typical.

d) At the time, coils were either wound by the hobbyist or purchased to a specific range. If no plug exists, then your hobbyist was uninterested in, or could not afford that option.

e) At the time, there was no 'set standard' for radio parts, connections, hardware nor much of anything else. The typical hobbyist, working on-the-cheap as this one clearly was, would use what was lying around. Which might explain some oddball stuff.

f) Note that the standard for *cheap* wire at the time was rubber/gutta-percha insulation. Which would crumble to dust in short order. So, PVC is no surprise.

On that one-handed coil - are there any taps on it? Commonly, a poor-man's SW or Airplane frequency coil would be done that way - one end fixed, and then tapped to the standard antenna for the alternate.

Summing up, you have the equivalent of a fox-hole radio created with as few expensive parts as possible, following the general idea of the Armstrong circuit, which was 'all the rage' at that time.

Peter Wieck
Melrose Park, PA