Beats the heck out of me. Some of the vintage pieces I've seen have idiosyncrasies that nobody seems to be able to explain. The old spoon design I modified had a couple of grooves on the concave side in the shape of a "V" and a single groove along one edge of the other side. I got rid of the "V" and left the other one in place just for the heck of it. A friend of mine has a similar spoon from the same era, (and also from the Westerly RI area) but from a different mold that his father had made - it also has the "V" grooves, but not the other one. We've racked our brains trying to come up with an explanation, but so far no luck.
