F77 made its debut with the Seventh Edition Unix. As far as I know it was one of the first Fortran 77 compilers.
F77 used the same backend as the C compiler, so it was pretty portable. F77 was also used as the basis of the f2c translator, which is used on systems where there is no Fortran Compiler.
Source code to the original PDP-11 Fortran can be found here.