After being sandblasted and primed, I noticed one of the damper units had suffered a lot more corrosion than the others. So much so, that oil fill plug had been completely eroded away. The plugs have a taller head than a standard bolt so I decided to repair the existing one rather than replace with something that didn’t match the original design.
I started by extracting what was left of the original bolt by welding a nut on to the end of the corroded lump. Once removed, the nut was cut off and then the end of the thread faced in a lathe to create a flat surface. I found a nut in the scrap bin which matched the hex size and had plenty of height to match the original plug. This was then cut to size and brazed onto the original piece of thread. Once screwed in it now matches the other 3 damper units perfectly!