Bill and Larry. Is it possibile that the shot with the racing no. 16 published on Freundenberg's book was taken in 1921?
In fact if you look at the 1921 Keig shot (that published on Savage's book) you find that there are many particulars that the portrayed motorcycles have in common. For example the tools bag and some cables on the engine - all things that on the motorcycle portrayed in the 1922 shot published on the BP-Shell book are different.

Of course another logical possibility is that the shot published on Freundenberg's book was not taken on the isle of Man.

P.S.
Larry, many thanks for your answer.