Wow, sterling silver tools... Wonder what the Mrs. would think if I put that on the credit card...
I have made a couple of little squares about like what you describe. The first one was simple, but quite functional. Made out of hardwood. I only had handtools, but was able to get the faces perfectly flat by sanding the legs on a sheet of sandpaper glued to a sheet of plate glass. I used epoxy to assemble the square (so that I wouldn't have problems with glue shrinkage and draw). I pegged the joint afterward with hardwood toothpicks, but that was just for looks.
Could have put brass plate on the inside edge prior to the sanding process. That would have given the little guy more longevity.
The small hardwood version was quite acceptable for squaring plane blades, etc.