I was wrong: it isn't. @ is not in the character set (except in character constants) for other compilers.
Whether or not WINIO@'s variable number of parameters is OK, WINIO@'s @ isn't OK. Some of the interfaces in the insert files may just not be standards-conforming, and certainly it is possible to use non-standard INCLUDE syntax in FTN95. None of the compiler directives, including WINAPP, will compile with lesser compilers.
So, Clearwin+ isn't (standard-conforming) Fortran - and you were right - but by heck it looks like it is!
Eddie