Right I understand this, but I'm fairly sure there is an official spec for Forth local variables somewhere.
Standard (ISO/ANS) Forth (1994) has a LOCALS wordset, which is meant to build more convenient syntaxes on, not to be used directly. The various locals wordsets people used at that time differed too much.
Nowadays implementations for locals have converged a bit more, and the upcoming Forth 201x standard will most likely include a locals wordset not unlike what Apple used, but slightly different syntax (and quite different semantics if you look deeply!)
Gforth has similar syntax, as do many other systems.
And, of course, local variables in Forth is an abomination that should be avoided -- just like ROLL etc. :-)
Segher