Brainy.
"SB643B is not even a 'mandatory' SB. It is marked 'compliance will enhance safety'"
So why even do the 500hrly then!? It's the same SB.
It also said,
"magnetos MUST be overhauled or replaced at the expiration of five years since the date of original manufacture or last overhaul, or four years since the date the magneto was placed in service,
Per the Reg's, The components must have an overhaul period, the SB sets out that period. Period.