hello leon,

obviously the horseman attachments are a bit thicker than the mamiya ones. I had the inverse problem to fix a horseman back to a mamiya 23 graflock back. the mamiya sliders wouldn't slide over the horseman back. by adding a thin washer under the sliders it worked, but then the mamiya attachments wobbled a bit. so try to add a tiny strip of hard plastic sheet between the back and the sliders, just where the sliders would go (upper side, not camera side). if it works, glue them. it solved my problem anyway. hope it helps.