They give the ETRHO on that label too which is 559x24 as best I can make out. ETRHO is pretty much the same as ISO. It gives the BSD as 559mm, and since this is on a rim, the 24mm wide across the inside width of the bead seat. On a tire the width part of the ISO/ETRHO would be the external width of the tire.

For me it is much simpler to think in terms of ISO/ETRHO numbers when looking for tires. My son has a 29'er. I wanted to get him some narrower slick tires for him to do a Century ride with me as he had no other bike at the time. I was told by many that a 700C tire would not fit that bike which the rim in bold had AT-650 on it.

The iso/etrho number on the rim is 622x20. So I went ahead an bought 37x622 tire which is also labeled as 28" x 1 3/8" x 1 5/8". It fits perfectly and he rode the Century on a tire more appropriate for road use and much lighter tire than the originals.
https://goo.gl/photos/yHueycTfET2nJzwg8