You are correct to conclude that @n
indicates the number of bytes for the function parameters. You can find the relevant MSDN documentation here and here.
In particular, that is how MSVC decorates extern "C" __stdcall
functions in C++. Note that this is technically implementation defined and other compilers do decorate this differently.
For example, MinGW-gcc decorates it without the leading underscore. Embarcadero's compiler(formerly Borland) does not decorate __stdcall
functions at all while Digital Mar's compiler uses the same decoration scheme as MSVC.