That 667 field was added to any authority record with a subfield $c in the 100. It doesn't necessarily mean that the heading is not a valid RDA heading.
The examples in 9.4 are used as additions to the preferred name. See 220.127.116.11.1. The heading for the princess is probably OK as it is.
I turn to the collective wisdom for help with a NAR for a Duchess of York, Frederica Charlotte of Prussia. I find the RDA rules for royals (see 9.4 and 18.104.22.168) to be extremely hard to parse. She’s a royal both by birth and by marriage (daughter of a king and wife of a king’s son). She also doesn’t have a last name in any real sense and the examples in 22.214.171.124 all involve people with last names. The examples at 9.4 only show the title and don’t show the whole preferred name.
There is currently a NAR in the NAF, but it has a 667 THIS 1XX FIELD CANNOT BE USED UNDER RDA UNTIL THIS RECORD HAS BEEN REVIEWED AND/OR UPDATED and I want to do the right thing and clean up the record while I am looking at it.
Frederica Charlotte Ulrica Catherina, ǂc Princess, Duchess of York, ǂd 1767-1820
Thanks in advance,