As with most porcelain insulators, there are distinct but settle differences between them. This is what makes it difficult to identify there U number's. Here is an example of what I mean. Left to right: U-292 " Hartford Faience" A-105 / U-293A "Porcelain Products" / U-314 " Westinghouse" All with "incuse" top dome markings.

Although there is no great value here, the thrill is in the hunt.