Rover 200 Rover 600
Can I ask why you want that particular bolt? I deliberatly used stainles steel botls with washers in order to prevent rust.
In post above I suggested that stainless stell should be used.Those part numbers appear to be NLA and have been superseded by DYP10035, which rather than being a flange headed screw, has a normal dome shaped head with a captivated washer beneath it.
I would think that a decent nuts, bolts and fixings specialist supplier would be able to source some flange screws of the correct design, although I would recommend using stainless self tappers and stainless washers - I have replaced the screws holding the arch liner to the front fenders on my ZRs in the past using the genuine MG Rover screws , but they were a mass of rust and unsightly again within a couple of years.