This striking triangular 1 franc postage due stamp from Monaco features a finely engraved depiction of the Sikorsky HO3S-1 helicopter of the U.S. Navy, showcasing early rotary-wing aviation technology. Issued as part of Monaco’s Timbre Taxe (postage due) series, it stands out for its bold red framing and detailed green line-engraving of the aircraft. The unusual triangular shape adds strong visual appeal and makes it a popular choice among both aviation and tax-stamp specialists. A classic mid-20th-century Monaco issue valued for its thematic crossover and crisp engraving.