The patridge front sight on your posted gun was usually mated with the elliason rear sight. Your gun has the accro rear and instead of the common ramp front sight has the patridge. It could have been ordered that way or change out along its 40 some years of life.
As above, Colt offered a special order factory installed Target sight set with the Patridge Target front and the Ellison Target rear.
When Colt installed one, they installed both, since the higher Patridge front often required the standard Accro rear sight to have to be raised so much, it stuck up too far.
Best guess: Some one replaced the standard ramped front sight with the Patridge.
The "FF" is an inspectors stamp if it's below the actual serial number.
These inspection letters were usually one letter, but sometimes, for unknown reasons you may see two letters.