Or is this a rumor, that the mag wells were modified so that only postban low capacity mags will fit?

Back some time ago, I posted about the same thing, in the "Magazines Forum".

While at a gun shop, we tried a pre-ban ruger mag (20rd) in a post-ban Mini14 and it wouldn't fit! This got our curiosity up, so we tried the same mag in a Series 181-XXXXX and it fit just fine.

I can't find the thread now. But, as I recall, most thought it was just a problem with the post-ban Mini we had. The mag would probably be OK in other Mini's

Jeez, all that sounds something like a DemoncRat would say. Alot of talking and no real answer!

I use preban Thermold and PMI mags in my Mini-14.

It is a rumor.  Several of us checked with Ruger themselves and they said that this had not happened and others tried preban mags in postban guns with no problem.

its a rumor when you think about it making a gun use a different magazine would be a big pain in the ass, they would have to redesign the old 5 round mag as well.