I have a Ford 501 sickle bar mower. The only way to adjust the ledger plate to sickle section clearance that I've ever heard of or done is by bending the hold down clips to hold the knife down near the ledger plate. I've never seen any shims to do this. I guess there could be some for under the ledger plate, but I don't think so.
Registration is done by adding or removing shims between the yoke bracket and the main frame. I don't have any way to scan a document at this time, but if you can get your hands on an Operator's Manual, the procedure is shown with a diagram to explain where to add or remove the shims.
Tom in TN