missing part

Noticed that the underside of my Prado is missing something when compared to other photos of the undersides, what is it that I'm missing and is it very necessary?

My 1/1975 FJ45 has had an aftermarket mirror fitted to the passenger side. I have been able to source a replacement, round type but it it is missing the ball/socket/ retainer plate with the 3 screw holes. I have had no luck finding this specific part and was wondering if they are obtainable or...