I don't have a Pre B CZ 75, mine is a Pre B CZ 85. Similar internals though for the mag. catch and spring.
That screw may have a very narrow slot in it. Many European guns use very narrow slotted common screws. I had to order a screw driver with bits from Brownells for my Auto 5 - and the screw in the Pre B CZ 85 required that same thin bladed screw driver.
The screw was also staked to the frame. Soak it with some oil each day for two or three days before you try to take the screw out.
The spring will come out easily once the screw is out.
The second (thin) spring is what puts upward pressure on the trigger bars on each side of the frame.
The mag. catch spring will "open" up when you pull it out. It takes a pair of needle nose pliers to squeeze the legs together and put it back in place.
When you tighten the screw down, be sure to check/watch the thinner spring that pushes up on the trigger bar. Too much torque on the screw will twist the legs and may keep them from lining up/fitting into the grooves on the bottom of the trigger bars. If not in the grooves the legs of that spring won't put the proper upwards pressure on the trigger bar.
On mine I tightened the screw down, noticed the spring legs twisted out from under the right side of the trigger bar and then loosened up on the screw just slightly. Not much, just enough to let the spring move back into position.
They are great pistols. I was just telling my wife this morning how much I like mine, as I put it in the holster to take the dogs for a walk. It was in pretty rough shape when I got it (many years in Israel with a good mix of fine brown dust and old grease packed into the corners and crevices inside and under the grips) and now has new springs, new grips, new sights, a new barrel and a cerakote job (as well as a frame bob to allow the use of some modified 16 round magazines (now 18 round).
And they are built like tanks. Had this happen about a month ago. Gun went bang, bang, bang, bang, bang and then nothing. I noticed the slide was hung back, saw a round jammed into the chamber mouth, racked the slide, and saw I'd had a "kaboom". I field stripped the pistol and look what was left in the barrel. Absolutely no damage to the pistol. Only way I knew the round came apart was that it failed to feed/chamber the next round. Oh, I don't recommend a similar event, just letting you know they are well designed, well made and strong as hell.