I am planning on making some homemade Etachron tools from brass rod.  This will be flattened and formed into a a flat crescent, with a slot filed into it that will act as a wrench for turning studs. Would steel be better? I could use pieces of old mainspring that I have annealed to allow them to be filed to shape. Does anyone know the dimensions of the studs?  Are they all the same? I know that there are differences between Seiko and ETA.  I only have one watch on hand with Etachron (an ETA based movement), I can measure this but I don't know if that would do for all ETA based Etachron.

Hello and welcome from Leeds, England. 

I made some similar probes out of 3mm brass rod.  Chucked short sections in an electric drill and did rough shaping against a file.  Followed by an oil stone and them progressively finer emory cloth.  Very useful tool for shock springs.

One available here, not too badly priced either https://www.ebay.com.au/itm/126925987414 Edit# sorry I see later that you have already this one in mind

Welcome! This forum is a much more helpful community than you will find anywhere else for watch repair.