Getting back to the original topic of this old thread, about Bugatti ever installing a spark plug on opposite sides of the cylinder block, I was very surprised to discover something interesting on page 102 of Barrie Price's book on the type 46 and 50.
If you look at the upper picture (I would have posted the image if I had the slightest idea what I was doing), you see a photo of the Type 46 prototype engine. Notice that it has only one spark plug per cylinder, unlike the twin spark production version. But- when you look at the distributor it has dual ignition and then when you look harder you notice that the other eight HT leads are all fed down a tube to a set of plugs on the other sideof the engine.
I have been studying the very earliest factory drawing of the Type 41 engine overview and I now believe that at some early stage of the prototype Royales life it also had this plug arrangment. Most of the part numbers are for the Type 34 and it really indicates where the design came from. One thing that is for sure though, the story of the prototype having a larger engine capacity than its production offspring is a load of nonsense, every drawing held at the Bugatti trust has the 130mm stroke.

Getting back to the very first point of this thread, if you look at the photo in the Kruta book of the railcar engine, sure enough it has double ignition.

What I can tell you as a definate fact is that the prototype Royale chassis was not longer than the production chassis- it was actually 80 mm shorter. The wheelbase was the same as the subsequent cars, but the distance ahead of the front axle proved to be too short to provide an adequate length for the front springs. A modification was made to the prototype chassis towards the end of 1927 which extended this distance to make it the same length as the production cars.
The engine for the prototype was also unique to this car, it did not have the elaborate camshaft clutch/bevel gear arrangement we see on the cars today and because of this, the front tower for the camshaft drive is more compact. Because of this the engine sump was some 50 mm shorter than on the production cars.
As for the bore and stroke of the first engine, it was identical to the later cars- the 14.7 litres that Bradley spoke of in 1926 was rubbish.
These are a couple of points that I have discovered after obtaining all the Type 41 factory drawings from the Trust.
Another thing that I have discovered is that on the Type 46 (the nearest thing Bugatti made to the Royale), if you look closely at the photo on page 102 of the Barry Price book on the big Bugattis, you can see that the prototype engine had double ignition, with eight plugs on each side of the engine in the manner that Jaap is talking about with the drawings in the Hucke/Kruta book, not 2 plugs next to each other as in the production Type 46 (and Royale).
One thing I can also say-there were seven Royales. The prototype was not the car we see today as the Napoleon. I dont even believe it carried the same chassis number.

OK, a very nice document.

Would it be possible to go through the list of cars mentioned, and check history for each of them. The claims Mr. Cesari makes could then have a tag:

-true
-plausible
-not plausible
-untrue

If we can trace a good portion as being true, the Royale story, which at this moment cannot be verified, can at least be tagged "plausible". And the big question then: Where dit it go?