Les Matheson wrote:

The slotted stabs were only on the slatted (post -556) birds. Most Es, all
Fs,Gs and subsequent models. Hard wing F-4s didn't have slotted stabs.
--

It sure wasn't part of the -556 mod (cockpit switches,
major rework of the armament relay panels, new wire
bundles, improved gunsight, and similar) but a whole
bunch of mods were done at the same time as -556.

I don't recall seeing *many* slotted stabs on F-4Es during
the period 1968-72, though there were a few. Originally,
the LE of the stab was without the fixed, mini-slats up front.
IIRC, the slotted stab mod had its own TCTO - but I have no
idea of the number. Finding it would provide a list of the
applicable tail numbers which hadn't already had it accomplished
on the TCTO issue date.

If I had to guess, it was around the time of the flash-
suppressor extension to the gun fairing, or shortly
thereafter... making it about mid-1971 or later.