Door mirror base

[Steve_D]

So the normal bit to break is the 'Toad stool' but has anyone found a source for the part below it that screws into the door? Or has anyone got one they no longer need?

We have a door where the base was corroded into the door but I have managed to cut it out and now need a replacement. I could get one turned up but have no idea what the thread is.

Thanks
Steve