Though it looks like Bob has deleted his original posts (though they still showed up in my RSS feed) I can confirm that Bob has the said RCA, and that Andy and I have been in touch with him. My email to Bob that he posted just gave an overview and was an initial correspondence. Andy followed up with a more detailed observation of the modifications.
Fred, the serial number is indeed curious. It's hard to determine if this is indeed the very first theremin (I had said earlier that I thought it was, but it may not be the case) but it is not a prototype, with respect to the modifications we can see and cabinet, at least. Thierry's right about the 6H6 and the other modifications, which were added at a much later date. The Hoffman-eque cabinet has parts from its original cabinet: the side rails where the chassis sits, and the sheet music shelf on the front.
Like Charlie, I think this could be returned to the original configuration, but would take a lot of work. I've got a perfect replica cabinet that I built that would make this a nice restoration job.
I have noticed that the theremin community here can appear to be a bit hostile when responding to people who have (or claim to have) and RCA Theremin for sale. There's good reason to be skeptical, since these instruments are hard to come by, and the seller might not be informed about what they have and the apparent/true value of the instrument. I've seen a lot of RCAs advertised here over the years. Some are sub-par restoration jobs that end up fetching a whopping price on eBay, while others that are in much better shape, though need some work (like the RCA that I bought), sold for much less. Then there are basket cases that some people on here have scoffed at and offered '200 bucks' for.
A few years ago this community unceremoniously removed a vulgar and rude man by the name of Billy who appeared to be trying to sell an RCA. Turns out he did have an RCA, confirmed with serial and door numbers. Don't know where the theremin is now, and don't have any way to get in touch.
Unfortunately, it's very hard to come by an RCA that's a good candidate for restoration that will be sold at a low enough price which, by the time one has put money in for electronics/hardware/cabinetry/finish, is justifiable.
The key, when you're trying to sell anything of value, is how you present the item and yourself.