THAILAND 1921,  (Mar 24), Experimental First Flight by Military Plane, Ubol to Bangkok, 3b postal card uprated by 3b adhesive,  tied by 1921 Ubol March 24 special airplane cacheted c.d.s. for inland airmail. Bangkok same day arrival c.d.s. adorns front as well, fresh and Very Fine, scarce, with regular airmail service beginning in 1922.