Cylindrical square from a lathe tailstock.

[tonyfoale]

For a cylindrical square that is already very nearly +- .0000 in every dimension get your hands on box of new old stock racing solid lifters for a big block chevy Ford Chrysler they wont be the largest but at about .875 to 1" by 2.25" if you are checking something not very large you can't beat one of them an added plus is they are polished to mirror and the hardness will be off the books

I have no idea where I got it from, nor why, nor what it was off but I found a gudeon pin 1" x 2.75" in my "waiting to be used box". It has a > 1/4" hole as Johan requires. It is beyond use as a square because I had used it for testing a tool post grinder.
A look through Summit Racing's catalog will probably show sizes and engine model for various pins.