I have one of these insulators, and yes it is for insulating a chair from a steel deck or look out platform. It is embossed
HARD MFG. CO. / PAT. APPLD. FOR
The one here is the one I have and I have had this one since I was 14 years old. This one was used at Mare Island Naval Base in Vallejo California. It came from a pile of broken discarded metal chairs that were removed from the elevated watch towers. The groove is designed to hold a spring clip above the base and there is a right angle bend that fit into a deep groove going up the glass. The clip would extend about 3/4 inch above the glass where another right angle bend in the clip was made to fit a hole drilled in the side of the chair leg, and thus securing the insulator into place. I no longer have the clip that was with this example or a picture would have been provided to show how it worked. Here are a set of photos in different angles to help visualize how this was attached and used. I would also like to make note here that the embossing is always worn from the installation and use processes.