Topps produced promo cards ca. mid-90s were often issued without notation and in one/ per order form sent to dealers OR in clear, cello wrapped packs containing the entire promo sets. As a former dealer, these were sent frequently to shops and were often accidentally moved into hand built sets, $0.25 boxes, etc, rather than being saved as "promo cards."
These are very known to variation collectors and are documented in several variation guides. Basically every set Topps produced for each sport on the mid-late 90s has unmarked promo or pre-production cards: Gallery, Bowman, Stadium Club, etc.
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