If I remember we have a main liquid line solinoid with a 20 lb loaded check on the right, beside control pannel for cool gas differential. This is the same on both med and low, they did alot of med temp cool gas defrost back then. The defrost slide valves are no fun to work on.
AD-defrost timer or controller
AL-low liq alarm
AS1-satellite comp #1
AS2-satellite comp# 2
older racks late 70's early 80's
A9- timer motor
Also there is high suction button on some racks.
AT- I think is low pressure
WTF- On most racks only between 2am and 6am
I have 3 of these racks at one of my stores, not exact model number but same era, same control strategy I actually happen to have a schematic I drew up ( the old school way) I'll try n scan it this week and send it to you!!
And as indicated al is low liquid level alarm, and at is defrost timer alarm..
Gosh that is an old rack. I havent worked on one of those in years. So far the guys are correct. I thingk too that the middle buttons were for each compressor alarm circuit as well.