My 1858 Pietta was ordered from Cabelas in December, so I suppose the date of manufacture was recent.
My cylinder slots are machined to exactly the same spot as jb in the above photos.
I have yet to find the Mike Beliveau video on how to do a modification to get the slots to work. Maybe he never made the video, even tho' he said he would.
But I do think he had the issue, so he may have, over the years found the reason for this.
He never answered my pm.