CZ 75 B magazines will not fit inside the magwell of a CZ 75 pre B pistol. The CZ 75 pre B magazines will fit inside the magwell of a CZ 75B pistol. I have both a CZ 75 pre B and CZ 75B pistols. The magazines for both are different. You need magazines specifically made for the CZ 75 (pre B). If your gun's empty magazines are not drop free when you hit the mag release button, you can purchase a drop magazine brake from czcustom.com and install it yourself. I did. But, beware, the original brake, a thin, rectangular shaped piece of metal in the grip has a piece at the bottom end that is attached to the frame and I had to break off the original piece to get it out. Replacing it with the new one is the easy part and the magazines will drop free.
Here are some pics of the different CZ 75 magazines.
1st here is the magazines that you need for your pre B gun. I believe they are made by CZ, not Mec Gar.
The CZ magazine on the right is made by Mec Gar and is for the regular CZ 75B. The left one is for the pre B.
The CZ made 26rd magazines will fit and feed in both the CZ 75 and the pre B pistols.
Here is an interesting bit of info. if you want to modify your pre B to fit and feed CZ 75B magazines.
Link to site