I had to go get some sandpaper. I had what I think is a brainstorm idea. I went to the small hardware drawers where the stainless steel square keys for motor shafts to hold pulley or gears tight on a round shaft. I bought 1/4 , 3/8 , and 1/2.
I brought them home and checked with a micrometer they were within 2 thousand or less of there stated size to me thats close enough to be used as gauge blocks am I all wet or not?
By the way cost around $5.
If it works for you, go for it.
Precision machined brass or aluminum set-up or gauge bars, are the choice for many.
The brass set-up bars, and the Kreg set-up gauge bars, available thru leading woodworking suppliers,
are longer, and span the opening in a router base.
Generally, the square steel pins, you make reference to, are short,
that one finds in some hardware stores.
You could also use the shank end of your drill bits, as a set-up gauge.
I have long used steel key stock as spacers for setup, spacing and gauging, only resorting to drill bits for those sizes that I do not have in key stock. Most of these key stock blanks that I havce are 12 inches long (full length), intended to be cut to length at the time of installation. I do have a set of brass set up bars from Rockler and agree that the softer metal is easier on any cutting edge that might come in contact with them, but I normally use the steel key stock because it is longer and easier to handle than the short little set up bars.
The 12 inch bars are available from MSC and other industrial suppliers.