The 1904 McClellan had a self adjusting rigging which kept the centerfire rig in the center so to speak. I have one that I ride on an old mexican horse and it fits him perfectly. My Hope saddle has what I believe is full quarter horse bars and I think a full rigging with rear cinch. I believe that most of the Hopes were centerfire rigged back in those days. However, my problems is not just slipping backwards but it will walk forward to. I live in the hill country of Texas where there isn't much level gound so this is a problem. The saddle does fit my big QH good, but with no skirts and a bare rawhide covered tree their is no "stick" to the saddle on top of the saddle pad.
I do plan on making my own hope saddle for my other horses someday soon so I am really interested in fixing this problem besides of adding skirts. I just like the look of the thing, though it isn't much for ranch work, when it's on my horses we look we just rode out of the mid 1800's.
Have a great day,
Vince.