Hi Chris,
Having had a look at your photos and those of my two mowers I do not think that they are different. I have a sneaking suspicion that the cylinder head and exhaust have been reversed on yours at some stage. This is very easily done as it is only a few nuts to undo. From all of the photos that I have ever seen of these mowers and the two in my possession all have the exhaust and decompression valve to the front and the carburettor to the rear.
If you have a go at starting your Centenary take care as it could take a few goes and be careful of the crank handle when the engine fires. Once running it should pull along like a train.
Cheers,
Sir Chook
