162P4 12c Henry Clay 1873 issue plate proof on card
The Continental Bank Note Company Issues 1873-1878
As Luff explains, the contract to print stamps was put out for bids in December, 1872, with the successful bidder being the Continental Bank Note Company. This firm took over the printing of stamps beginning in May, 1873. This would be the firm which held the contract for the printing of the first proofs in 1875.
The first issue of large bank note stamps which were created as card proofs was Scott 156-166. These were created in colors which were the same as the issued stamps, with the exception of the 6, 24 and 30 cent stamps. Even these colors were close to the issued colors, being pink instead of dull pink on the 6 cent stamp, violet instead of purple for the 24 cent stamp and gray black instead of black for the 30 cent stamp. Despite holding the contract for five years, only this one issue would be created by the Continental Bank Note Company. 30 As noted by the editors of the Scott Specialized Catalogue of United States Stamps and Covers, there were secret marks on the 24, 30 and 90 cent dies used to create the die proofs, but no plates were made with these dies, so these secret marks do not appear on the plate proofs. 31 Although the Continental Bank Note Company will cease to exist in 1879, the proofs from this series were evidently prepared and printed prior to the merger with the American Bank Note Company.