The special cancellation date is correct, according to the Bochmann catalog. Michel doesn't make note of this.
The Michel catalog does, however, make note of strange dates for the Robert Koch stamp which was issued a few months later on Jan. 25, 1944. With this stamp, Michel notes "(The stamp) was sold starting from 25 Jan. 1944, however some were cancelled to order on Dec. 11, 1943 (Koch's birthday)."
So, I'm thinking the same thing was done with the 1943 Brown Ribbon stamp. They just cancelled some a few days early, on special covers.