I found these at barn sale. There was a whole box of them—apparently new old stock. Each piece is identical with 1706 recess-embossed in the middle of the inner curve. They were apparently designed to go together, and they fit properly in only one direction, with the lower right photo showing them mated incorrectly. When they are mated correctly (center pics) the unglazed firing rests on both pieces are on the same side. Anyone know what these are? Common? Desirable? Thanks for any info.
UPDATE: Thanks to several folks who responded with information:
Richard C. said, "This is a cable insulator for a street light. The cable goes to the street light and is split to install cable." He added that these have "many other uses, too, but I think this is what the main purpose was." In addition, he noted that "this insulator was in a metal clamp and was fastened to the pole. Just [as if] you were grabbing a wire in your hand, and your arm was the bracket. The wire went through the insulator which was your hand." Here is an example to illustrate: [id=204252950].
Richard W. offered, "These look very similar to what I've seen in use in buildings under construction to temporarily support electrical wiring, conduit, etc."
Dave K. has one on his living room floor. He said, "They are used on a rack: the bottom one insulates the cable, and the top is then put on the wire. A U-bolt or strap is then placed over them and then bolted to the rack. Not especially pricey, either."
Thanks for the help Richard, Richard, and Dave.