A bit of googling and amazoning suggests to me that the last true single volume edition was about 1943. But at least through 1955, the two volumes (USA, US Possessions, British Commonwealth in one, the rest of the world in the other) were being manufactured and sold in a "combined edition"--so in terms of actual physical single-volume, at least to 1955--which is getting pretty close to your 1963 two-volumes.
I saw copies of these combined editions available on
Amazon for not all that much money.
Edited for typo