I don't think Champion makes auxiliary gap plugs anymore. U: auxiliary gap, J: 3/8" 14mm thread, 10: heat range, Y: extended tip.
All U.S. (Champion, AC, Autolite) plugs have lower numbers for colder plugs, AC 44 is colder than 45 which is what you'd expect for a heavy chassis. All foreign plugs (NGK, KLG, Bosch, ND, Lodge) run the other way.
AC is the original plug, and IMHO is always a safe choice. Other brands may be listed as the equivalent in a chart, but that only means "closest match of what this manufacturer makes", not absolutely identical.
I have some general remarks on plug heat range on my page:
http://victorylibrary.com/tech/plug.htm
