Cylindrical square from a lathe tailstock.

[Toolmaker51]

Those old low speed inline engines, common in pumps, stationary plants, some autos, [long cast iron pistons, low pins] would be excellent squares. Low center of gravity, free of charge.

[DIYSwede]

I'd expect wrist/ gudgeon pins being very square by nature [to length as well], with expectation retaining rings or buttons keep them located. Lapping ends won't degrade hardness, even case-hardened.
Just visualized über simple lap fixture. The proper connecting rod, provided crank and pin end are same width, not twisted of course. Certainly square by design. With a hand motor, relieving [grinding] bottom into 3 pads would be easy.

Sure - but for smallish ones used for squaring purposes on, say a face plate or a mill I'll need them cut in half, thus the lost hardening.
Here I simply assume they're not thru-hardened by the case hardening. I'll get a matched pair from each pin, though...
After initial checking for true roundness, I'll probably make an adjustable soft aluminium lap for the diameter, as per Tony's design-
carefully putting my handle so it won't self-tighten on the pin and lock on me.

For checking their axial trueness: just a simple setup as per Franks suggestion:
Clean and drop three of them on two long axles (held together on the surface plate), both axles grooved at the centre pin's ends.
First one against a ball bearing end stop, a mumeter indicator at the third's farthest end.
Turn just the middle pin and check indicator, tape 1st and 3rd pin lightly to one axle if needed to avoid their rotation.
Clean, flip, change and swap ends 'til all surfaces have been checked - 6 runs would suffice.

Then, if found necessary: lap ends thru finer grits in the same way but swap indicator for spring loaded ball bearing.
Rinse and repeat 'til satisfactory trueness is reached. Think I'll stick with circular end surfaces - not 3 pads.
Then cut, grind, relieve and lap the fresh (unhardened ends) of the short stubby ones. Mark for pairing.
Did I say: -"Clean meticulously between each and every step above"?

[Toolmaker51]

All your initiatives are correct. Again, I'd think the size range of internal combustion engines broad enough to supply pins in any proportion desired.
The pad idea reduces contact without any loss of stability. If I'm doing it, the old felt pen marks are fine, removing about 25% of the area.
Lapping: Quick results on wet glass with fine wet-or-dry [also wet + drop of detergent] silicone paper. Hold the pin down with three fingers. BTW; lapping is best done somewhat elongated figure 8's, not circular. Then, straight in one short stroke.

When you see crosshatched 'tool' marks, it is pretty damn flat. Not unlike honing an engine cylinder, seeing the resultant 45° marks.

Hence, I'm endorsing meticulous clean and rinse steps fully. More intense the better.
Then I'll research Swedish companies producing those supplies and purchase shares of stock.
Lol

[DIYSwede]

A quickie (10 minutes) demo on Cylinder Squares by Keith Rucker,
featuring a Brown & Sharpe No. 558 and a Wide Base Magnetic cylindrical square.