I recently bought a few machinist chests full of tools and other goodies. I'm not a machinist myself, but I'm familiar with a lot of the tools associated with the trade. This piece is a smooth solid steel cylinder with four equal spaced holes bored completely through it. It measures approximately 5.5" long and 2.25" diameter. Weighs almost 4-1/2 lbs. The end has stamped "Feb. 48 Eddie Champ." The other end is identical minus the engraving.
I suspect it might be a cylindrical square, but I've not seen examples quite like this. It came with a plastic sleeve.
I think it's a cylindrical square. It can be set on one end on a surface plate to find 90°. Because it can be rotated, it can be checked for error. I think they're often made by apprentice machinists, hence the name and date stamped in it.
Example commercial version: https://www.mscdirect.com/product/details/06504302?item=06504302
Example use of one with a surface gauge as a squareness comparator: https://youtu.be/53q6kVX9gjM?t=1244
I'm pretty sure it is a cylindrical square and was someone's shop project in tradeschool or as an apprentice, or just from a slow day.