Strange one...clearly stamped '20' on the baseplateNoticed this on the C12 I removed from the Montreal I picked up a few weeks back.
My main camera's being serviced so the photo was taken using my Iphone with a loupe attached - so, maybe not the best quality :-(
Also visible is the standard 'SX' and 2 bars on the edge just below and left of the '20'.
I mentioned it over @ OTD here
Arno replied saying he'd seen one also in an Autavia 1163 MH with a much better photo in his book, pages 66/67.
First thought was maybe a high beat prototype, however I clocked it @ 21,600 vph using the iphone app Kello.
So, another Heuer mystery :-)