The fact that it includes the sheet code tells you all that you need to know.
Topps promos and pre-production cards do not contain them with the very occasional exception, ie, ABCD. The do not make three different complete sheets of promo cards as would be the case in order for Brett to receive a C designation.
This card likely came from the Leximo’s seemingly endless supply of sheet cut “oversized proofs” of junk era cards, which you can find many similar examples of on eBay. All either have the white border or measure larger as the base card.
https://rover.ebay.com/rover/0/0/0?m...2F352642994055