O-ring #01-015. The first two numerals refer to the material, the last three to the size. The 01 is code for nitrile rubber, 70 durometer, the 015 is the size code for 9/16 inside diameter (nominal) by 11/16 outside diameter (nominal). If you have an industrial supply house handy these will likely be cheaper there than if you obtain them from an automotive supply concern.
The outside diameter of the Lyman sizer die is 0.720" at the top, and 0.700" below the o-ring groove. The groove is sized a bit over nominal tolerance to allow use of this ring, a common practice when part sizes aren't vulgar fractions (the o-ring will stretch, seal-off is no problem because the rings are actually about 0.073" thick on average reather than the 0.0625" that you'd expect form the specs.