U-290 thru U-299
U-290A's these are all dry processed
Just a good look at dry process "crazing"
similar to a U-290
I read about this odd marking in an old issue of Crown Jewels (I'll try to add
issue info asap) in Jack Tod's articles "Porcelain Insulator News". Mr. Tod
said something along the lines of this being an impression of a standard screw
head from the manufacturing process.
similar to a U-293B. Love those darker blue specks.
Both are Thomas products.
If I had to venture a guess I'd
say these are all Lapp products
except for the yellows which
may be Westinghouse products.