Ahh, McMaster! A coworker gets materials for his art work there (he's an engineer that likes to weld) and showed me around the website when I was looking for some copper sheeting (before I discovered shim steel).
Since Amazon also has stainless shim steel, I don't doubt the shim steel I bought is not. Oddly, US Steel describes carbon shim steel finish as "either a matte finish or a smooth ground finish," but I agree that the shim steel I have sure looks shiny like SS. I'll have to keep an eye on it for rust, but for a 1/2 oz. windscreen that won't melt, I don't care if it gets a little rusty.
The holes were punched with my favorite hole puncher: a super-cheap child's hole-puncher that was part of a set (stapler too) for 99 cents a CVS. It works better than my "heavy duty leather punch" and will even punch thru steel cans! The spacing is 1/2" o.c., and the offset is based on the max. depth of the hole puncher, about 1/2".