As Jethro says, the external fixing points are identical for 400, 500 and 650 RFVC lumps.
I know I shouldn't have, but I started the bidding. He says he has loads of watchers so I'm confident that I won't have to hide another Ebay purchase away from Mrs Onepot

Mind you, that's exactly what I said before I ended up buying my GB sidecar outfit

GC