What engine is this?

Can anyone out there identify this engine?
It’s presumably of aeronautical origin, which is why I’ve put it before this forum rather than a veteran motorcycling one.
The bike is a 1912-vintage French Verdel, and is usually based at the Sammy Miller Museum in Hampshire. It was brought up to the Thundersprint meeting in Northwich a week ago, which it was photographed.
Sammy Miller’s team seemed to think it was a Gnome, but they weren’t all that certain and besides, I though Gnomes of that age were all rotaries. Now that would have made it interesting to ride!

William