Hello, is there someone who knows about old windmills/old windpumps.
I have a Horwood Bagshaw/Metters over a bore and the pump rod that
extends out of the bottom of the gearbox is worn nearly through from
scraping on the side of the gearbox casing. I have spare steel rods
and so I need to know the type and size of the thread at either end
of the pump rod, for instance: 7/16 or 1/2 inch, and whitworth or
bsf etc. thanks.

Not sure this is of any help.
I measured 2 pumps, and the pull rod nuts are different, one is 5/8 18 UNF and the other 5/8 11 UNC
ones for sale are mostly 5/8  BSW
They might be southern cross, I do have a windmill in parts but it is a different brand, the others are Jack pumps similar to those in the document pg 58. onwards.
http://www.gvps.com.au/Tech_specs/Windmills/Metters%20Mills%20Pumps%20Pumping%20Gears%20Troughing%20Squatters%20Tanks%20Tank%20Stands%20Etc%201944.pdf

Thanks Aaron that's helpful, and good .pdf document.