This cable top style appears to be similar to U-447C. Please correct me if I'm wrong; porcelain is not my area of expertise.

The embossing is the letters "PP" / "-INC-" in a square, with the year, "1956" underneath. It appears to be ink stamped on the piece. Interesting!

This is one of a few interesting porcelain insulators I was lucky enough to have in my possession for just a few days before delivering them to their new owner. I liked them so much, I decided to photograph some of them and post them for reference.