Originally Posted by Twin-Stack
My FROG 50D Mk. II...
For Bob and GH,
I think there's some confusion about the Frog 50 Marks.
The Mark I is identifiable by its slightly different lower profile head, which is clearly shown in GH's photos.
The Mk II has the same crankcase, still with the perpendicular NVA, but with a slightly larger, and slightly different shape, head.
The Mk III is easiest to identify, it's the only
one with a swept back NVA and the "stroking" band around the crankcase.
So Bob, the engine you think is a Mk II, for me, is clearly a Mk III.
One other point, these Frogs never had "original" radial mounting plates, but all the front rotary Frogs were said to be radially mountable just by removing the tank and using the backplate retaining bolts. That's why these bolts are longer than apparently necessary, although many have since been shortened (to get the engine snug up against the firewall when using a separate tank).