According to the information I have, what used to be called double proof is now called triple proof, with the proofmark repeated 3 times. If your gun dates from the 30's, then that's not 11,000 kilos . . . but still a very stout proof at 18,000 psi. The Robust is more or less the Stevens 311 of France in terms of numbers produced . . . but much higher quality. Very solid guns. "Robust" is a well-chosen name.