These markings told handlers: "Transport this letter by surface mail (train/ship) only as far as the specified air gateway. From that point, it must be transferred to air service."
They were applied when mail traveled by air for only part of its journey before being transferred to surface transport (train or ship) for the remainder. Jusqu-a Airmail Markings- A Study Ian McQueen
Have you found a Jusqu’à marking not listed in McQueen’s original index? Contact the Air Mail Society’s literature bureau—they are currently compiling an addendum to the late McQueen’s masterwork. These markings told handlers: "Transport this letter by