Ok, maybe I have missed something here but my speedo and light cluster is like this:
It mounts into threaded holes in the upright of the 'U' shaped part of the headlight bracket and to fit the rev counter assembly you simply remove the two 'pin type' screws (of which Nasher made some excellent Stainless Steel versions) holding it in place, fit the rev counter assembly over it and secure.
There should be some adhesive foam on the return edges of the rev counter assembly to prevent any untoward rattles against the plastic housing of the speedo but as Utopia rightly says- mine's been fitted for almost the age of the bike without any issues other than the slightly erratic dancing of the LED's themselves, which could be calmed down a bit if I could be bothered to.