What should be the actual official registry grade?
When a note that is already listed on the wiki comes back onto the market, the new information (ie. grade, location, etc) is listed with the old information and both are visible to anyone viewing the registry. So if someone looked at the Wiki, they would see that this note had been graded by BCS as AU-50 and now as a PMG AU-55.
On the wiki there are no "official" grades, just opinions of the seller or a TPG. A buyer should always do research about a note they are buying to find this kind of information. (ie. grade discrepancies)