Just short of $50.

Did you notice how the bellow box is somewhat deeper on the keyboard side?

I hadn't noticed it before, but you are right. Maybe there is some sort of air-agitator contraption in the right hand side; I mean, some of these old instruments could get pretty strange. I don't know why else there would be such a difference in width between the two end of the accordion. I am more intrigued than ever!

remembering the sound of the instrument, it could very well be a gated fan, like Chris guessed.
It sounds a bit like a Lesleybox on a Hammond organ,

On further thought, two problems with the fan guess:


1. Wouldn't the fan have to reverse direction with the bellow changes? The air flowing into the the accordion would turn the fan one way, air flowing out would turn the fan the opposite way. Seems way too clumsy to me.

2. The draw notes "suck in air" from outside the accordion and would not be as affected as the push notes which uses air inside the bellow.

I'm sticking with the "hidden reed block" theory with the back stop operating airflow into the off-tuned reed set.

I just couldn`t wait, so i sent an email to the man mentioned earlier on.
He actually owns and plays an instrument like that.

when he unveils the secret i`ll post it here

I wanna believe it may be some kind of tone chamber.