What we have here are two insulators manufactured by the R. Thomas & Sons, Co. They have applied metal crowns (that is, the metal crown is cemented right on top of the normal porcelain crown). This was to combat RFI (Radio Frequency Interference). The multipart styles that sport this unusual and neat feature are: M-2760, M-2940, M-3341, M-3365, M-3455, M-3623, M-4335, M-4451, M-4618. (If I have missed any styles, please send me a note). One day, I hope to have an example of each style.