The pre B gun mags were Mecgar.
Don't think so.
I was a senior moderator on the CZ Forum for a number of years, and this was a regular topic on that forum for hundreds of participants during that period struggling to find good mags for their pre-Bs.
As far as I know, CZ made their own mags for much of the pre-B production period, and then farmed some of it out to various makers when they couldn't keep up with the demand.
Mec-Gar didn't become the OEM provider until relatively recently -- (2002-2003?), long after pre-B production had ended.
While Mec-Gar-made mags look identical to CZ factory mags, looks can be deceiving. I've had CZ "factory" 10-round mags that had "Pro-Mag" markings on the paper inserts in the bags, that were identical to other CZ factory mags. Mec-Gar CZ factory mags appear to be made to slightly different specs than the after-market mags made by Mec-Gar for the CZ.
I've had many CZs, including a number of pre-Bs, and have used a wide variety of CZ-type mags (ranging from RamLine, Pro-Mag, Tanfoglio and Witness, Magnum Research, Mec-Gar, etc.) in all of these guns. Most pre-Bs will not accept Mec-Gar 15-round mags, but will accept the 15-round mags that were shipped with the pre-B guns; they will accept many of the Tanfoglio mags, and will accept nearly all 10-round mags.
This is more of a GUN issue than a Mag issue, as the mag channels (for want of a better term) in the grip of the pre-Bs in the older CZ s
are narrower at the top. Some late pre-Bs, called "transitionals," because they had some features of the newer guns, may accept either type of mag.