From the description you gave on inserting the Pro-Mag, they sound like a pair of National magazines I bought for my TZ a few years ago. They were $11 a piece so I thought I would give them a try, they're now paper weights. The top of the mag would bind on the trigger bar as it traveled the last inch or so in, the top of the mag didn't have enough taper to clear. After some pressing and filing to get them to insert and drop free they would not feed or cycle. Cheap spring, poorly molded follower and a rough surface inside. So they became paper weights and dust collectors for a while. I think I threw them away a few months ago, but I'll see if I can find them anyway.
Now, the magazine conundrum is about to get stranger. This may just add fuel to the fire instead of putting out the flames of "which bleep-in mag fits my gun".
All 10 of the magazines I have for the TZ75 and CZ75B combined fit and function in both guns. They all feed, drop free, lock in place, clear the trigger bar and lock the slide back. The only one that's different is the factory TZ mag, it will actually touch the CZ75B ejector if you hold the magazine flush from the bottom. But it just barely comes in contact with it, the mag clears the TZ75 ejector just fine. All the other mags clear the ejector in both gun.
I keep testing the mags in both guns thinking some age will start to show some problems, but haven't come across any yet.
Here's the list of what I have right here.
1 - Factory TZ, with the TZ75 when I bought it, holds 15 round, flat base plate and a matte blue finish.
4 - Mec-Gar MG-GT21, 16 round, flat base plate, polished blue finish. (these were bought in pairs about 3 years apart)
2 - Factory CZ, with the CZ75B when I bought it, 16 round, flat base plate, polished blue finish.
2 - Mec-Gar, "MG-CZ75-9MM" stamped on the side, 16 round, flat base plate, polished blue finish.
1 - Mec-Gar, # MGCZ7519 at Mec-Gar site, 19 round, Plus 2 plastic base pad and the new AFC finish. (looks matte black)
The 2 Mec-Gar mags for my CZ75B look just like the one below, product number 918412 at Midway USA.
The 4 Mec-Gar (MG-GT21) for the TZ75 still have the Assault Weapons Ban warning stamped on the front. "law enforcement, govt use and/or export only" I was going to supply a link to them but couldn't find anybody that had them in stock. Checked with Mec-Gar and concluded they were probably discontinued when the part number didn't return any results.
The MG-CZ75-9MM is the actual number stamped on the two Mec-Gar CZ75B 16 rounds mags I have, but I don't think it's a part number. I thinks it's just a description of the weapon it fits. The Mec-Gar19 rounder has CZ-75-9mm stamped on it, again I think it's a description. The actual part numbers were on the packages and I tossed the packages long ago.
Here are some size comparisons, if you have a set of dial calipers it may help in finding some magazines.
Factory TZ75 - mag body 1.25 x .810 (measured in the center of the mag body, from front-to-back x left-to-right)
Factory CZ - 1.249 x .808
Mec-Gar for my TZ - 1.258 x .795
Mec-Gar 16-rd for CZ75B - 1.255 x .798
Mec-Gar 19rd for C75B - 1.255 x .802
Something I noticed at Mec-Gar is that they now have 17 round 9mm magazines for both the large frame and small frame witness. One is number MGWIT9SF, I'm assuming the SF indicates small frame. The other 17 round 9mm for a witness is MGWIT9LF, thinking the LF is large frame.
Another oddity from Mec-Gar, note the number on the magazine below, MG-GT21-16. Photo from Mec-Gars web site for part number MGCZ7516. Could just be the wrong photo though.