Just got this after reading that sp01 mags should work...

https://www.midwayusa.com/product/1371161853/mec-gar-optimum-magazine-cz-75b-85b-sp-01-shadow-9mm-luger-steel-anti-friction-black

In short it will probably function but it's a tight fit and nearly impossible to remove by hand...

Cz factory 18 round for the SP-01 will work in a Pre-B. All of the Mec-Gar will not unless you want to file the frame down.

Greg Cote LLC sells CZ Pre B 16 round magazines, too.  And the shipping (for me anyway) has always been way faster than CZ USA.

greg cote llc cz-factory-mecgar-magazines cz-75-preb-full-size-16-rd-9mm-factory-magazine-11114-p-1403.html

Or, you could go to the CZ Forum, take a look at the thread on modifying the frame on the Pre B (its' all internal and when you're done its' invisible to the eye) so it will work with modern (meaning MecGar 16 round magazines you can sometimes get on sale for $18 each) 16 round magazines.  I modified my Pre B CZ 85 last year and bought a bunch of the 19 round magazines for it.

Pre B frame work to allow use of modern magazines.

I was having my frame and slide refinished anyway so now you can't see anything on mine even if you look inside with a light.