I was able to find another picture of the Royal Enfield Edinburgh factory. This one shows the cradles for the Flying Fleas, but also shows Calton Hill in the background. There are not many places in which one could get such a perspective of the Hill. In particular note the little dome in the middle. It appears that the best location for such a shot would be Greenside Row. In particular at the end of that street is a modern industrial building, suggesting that it could have been a factory in the past as well. There is also a vacant lot nearby, now a car park. One can "walk up to the building" using Google Maps street level view, but the perspective of the photograph is more likely from an upper level section of the building. From street level one does not get to see the dome, at least with the vegetation that is present today. Reader Mauro W. in the Royal Enfield Yahoo bulletin board, who knows Edinburgh, claims that this interpretation is quite plausible, since one cannot get views of Calton Hill like that from other locations. Will we ever know for sure?