The buckle on the sling in the picture is flat on the bottom and half round on the top. It was researched by Tim Parsley who makes Civil War buckles. He claims that it is an exact replica of one found on an original sling. It could be possible that more than one manufacturer made buckles for the English sling makers. I always try to remember to never say never or always when it comes to Civil War equipment especially when dealing with the British stuff. It is possible that both buckle variations were used.