Here is one of my favorites and an unknown. The ones on the left are from Parker McCrory Mfg. Co. Kansas City, MO. The little knob is marked PARMAK. The one beside it is marked in the inside Made In U.S.A.. The next one is a special design. I replaced the screw eye but the holding plate has a patent No. of US 2,450,730 issued in 1948 to G.Hord as a means to quickly attach an insulator to a rod. It's the only one I have ever found and as you will see in the next picture has a unique star pattern on the top. The pieces on the right are all marked either USA or Made in USA. I originally thought these were Dare Products items but have yet to find who they were made for. I'm pretty sure these were made by Wisconsin Porcelain Co.