Connect a lantern to lamp post

[Peter4443]

Hello

I have acquired an old cast iron lamp post that has a lip around the top of the post which has an internal diameter of 3.5 inches. Does anybody know what kind of fitting I need to enable me to connect a lamp post frog bracket to hold a lantern, also where can I acquire one please.

Due to the lip around the top of the post standard frog bracket will not just slide onto the post, any suggestions would be most appreciated.