Item 57520 and Item 75903 seem to by different. While 75903 runs smoothly with
„ Part 57518 Technic, Link Tread Wide with 2 Pin Holes“, 57520 has more narrow
slots. The bars from link thread stick in the slots, so running gets kind of
bumpy. There should be two catalog entries give the choise to get the needed
part.
Sorry for my bad english.