Turns out the exact replacement cabin floor fitting for the 285 is a CF-155 from Midland Chandlers. It is the cheaper chrome plated casting rather than the larger and better quality Aluminium CF-163. Even then it was not exactly the same as the original, having a slightly different internal taper which required quite a lot of abrasion before allowing a reasonably tight fit for the chrome post. Midland Chandlers were very helpful.
Cheers
Bob
Teal 285/74