I knew I had a reference to these at home so now I can confirm that the proper name is a Sykes Flange Joint.    Early insulated joints were made from a solid block of wood such as Hornbeam but these were not popular in the UK. The common method in the 1920s was to plane 5/32" off the top and bottom of a standard fishplate and use a 3/16" pressed fibre insulator between the fishplate and the rail. The fishbolts were insulated from the rail by using fibre collets in the rail holes. These fibre p