It might be possible that when Ford service mechanics replaced the wiper blades, they installed new 15" wiper blade as opposed to loading a 16" insert into the existing 16" blade holder. The reason for this is that during rainstorms, there is always a rush to replace wipers and the 16" blade inserts may have been in short supply. The only alternative the Ford dealer had was to install a 15" blade and then install a 15" insert, which was in stock.
Since my blades are OEM Trico with the correct stamps and codes for a mustang, and my car was originally delivered to the Seattle district, it makes sense. Even an original owner would not know or remember the change, or notice the difference.
Again, I like the look of the 15". Would a judge deduct points for a 15" on a 1969 Mustang in a concourse Mustang event?