Certainly seems the rubber to which I refer is a home’un. I too have never seen one before, but then again I’m much more familiar with BSA singles than their twins. Point is that when I remove the tank, that rubber bush invariably comes off the nut and stays in the tank tube – meaning the frame nut & lug beneath must be entering the tube to some extent. Which in turn means we’re relying on the top bolt rubber to centralise the tank. Given the tank tube is not that large, to my mind that offers the possibility that the tank could rub on that nut (especially the locktab below it, as the latter is larger in diameter).
Dunno if anyone has ever experienced problems with a tank rubbing in this area (as stated in my original post, I’m aware of the top tube rubbers, the anti-roll rubbers and the bolt rubber) but never the less it does offer a secure location because I can feel the tank locating on that bush when lowering it onto the frame. Which is why I’d like to do the same on my project bike (which also has an A10 frame and tank).
Minor matter in the scheme of things though, just another ‘what do we do here’ on my long (but thankfully declining) list of issues to resolve. Have just finished sorting the throttle, choke, mag and decompressor cables and associated levers. When CWC deliver my front wheel rim I’ll build that and then it’s only a few bits n pieces before it’s ready to go on the rollers - which I’m currently rebuilding to get more wheel grip. If the bike’s a runner then it’s only left to sort mudguards, a seat, footrests and brake pedal, battery and wiring, an exhaust system, and ………. Good job I’m not counting the hours on this bike build. Will upload an update photo when it’s on its wheels.