Dutch Bike Spitz?

I have now read a bit on the patent, and it is for a frame design that has actually been used extensively by Sparta, and it seems your Spitz has the same; a single tube that forms the seat, down and top tube and that has been squashed where the bb and head tube go.
 
What is the purpose of this part on the rear wheel?
At first I thought it was a rear brake, but there's no lever.
With what looks like a thumb-lever on the right side could it be some sort of rudimentary security device?

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I have now read a bit on the patent, and it is for a frame design that has actually been used extensively by Sparta, and it seems your Spitz has the same; a single tube that forms the seat, down and top tube and that has been squashed where the bb and head tube go.
Ahh thank you. I’m going to date the bike as 1974 because of the date of the patents and as they clearly didn’t go into mass manufacture or marketing
 
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