Any idea what this is ?

[P.V. Motorcycles]

A look at the show reports for the end of 1911 (November) indicates that the enclosed ZE-type Bosch magneto was being fitted to Show models and offered for 1912.
I suppose the serial number of the mag would give a clue if it's original or not.

Useful to have the original poster chip in - do any of the cast parts have 'REX 1912' on them, and is it the 499cc or 532cc single?

[R]

The 1911 and 1912 models were very similar; the illustration below is 1912.

The problem with the 1912 pic is that it clearly has REX cast into the driveside crankcase, in horizontal format.
And the specs say the magneto was chain driven...

https://www.flickr.com/photos/flattank_motorcycles/6142926538/in/photostream/