Guidelines are guidelines, not immutable laws of nature. You have found an excellent example of a sensible name that cannot be easily mapped into the normal Java naming conventions. So what should you do?
Make an exception.
Of all your suggestions,
Iso8859_1 is the only name that clearly and unambiguously expresses what the class is about. The various coding guidelines are just heuristics to find clear names. Here they fail, and you would do well to ignore them.
Checkstyle is configurable. Either adapt the config to allow such names, or except the class declaration from the name check. Make the tool help you, instead of making yourself slave to an incompetent tool.
(Rationale: The default Java coding conventions suggest the use of camelCase to visually emphasize word boundaries within an identifier. Underscores are used as word boundaries in constants because constants should be all-uppercase. Here you need a boundary between digits. As ASCII digits don't have case you can't use camel case here, and by elimination the only remaining option to separate digits in a name is underscores.)