This is the rule:
basically, a class name must:
-- start with a letter (a-z or A-Z) or underscore, or
start with a dash, followed by a letter or underscore, or
an escaped character (what you're doing);
-- be followed by zero or more underscores, letters, numbers (0-9), or dashes.
In practice, starting with a dash is almost never done, except by browser-specific extensions and internal stylesheets. As Beverleyh points out, I would not expect consistent/reliable browser support.
ditto for starting with an "escaped" number, except that it is far easier for things to get messed up. (Your situation, with unexplained leading zeros, is a good example of that.)
I would highly recommend starting with an underscore, followed by your numeric class name.