It's known as a carrier circuit insulator, designed for high-capacity telephone lines. The pinhole is smaller than usual because it's used with a steel pin. These are a pretty late item, from the 30s to the 50s or so. The 15-40 on yours indicates it was made in 1940.
These turn up on Ebay and such with some fanciful stories about them as being from the Civil war days of the Confederacy, but that's not what CSA stands for in this case. The carrier styles can be found embossed with codes of CSA, CSC, and CSO. The usual assumption is that C is for carrier, S is for steel mounting pin, and the final letter is some sort of design indicator, but I haven't noticed much difference between the A, C and O. They're CD 128 if you're interested in that.