They are not slats but are kreuger flaps. They do not extend downward from the wing like other flaps. They fold in/out. IN they are flush with the underside of the wing.

I will have to check the manual to see when they extend but I believe they come out with the initial flap setting of 1. The 727 had 1, 2, 5, 10, 15, 25, 30 and 40. Later 40 was blocked off for State III Noise and I understand 30 is also now blocked off on most. (FWIW, on the -135 the max flaps was 50deg