Haha we have two different things being discussed here it seems.

1) OP is asking if factory OEM magazines were ever made or offered beyond 5-rounds. I think we can safely and accurately say no, never.

2) magazines later modified to 6 rounds were very common in decades gone by and yes, it would be a little crazy to make a conscious choice to do that now

I wouldn’t pay market price for a magazine now that has been modified — but that’s not really saying much because I will HELL NO pay current market on 52 mags anyway. But I sure wouldn’t be afraid of a modified magazine, I would just want to see the modification reflected in the price.

As a side note, Mec-Gar has a blurb on their website that tells you to suggest products you’d like to see. I suggested these. They were very quick to reply “sorry, we have no plans to do that.”

Frankly, I think they are out of their damn minds. Look at the market, and these haven’t been made in YEARS and Mec-Gar could price these damn things at $60 each and the public would buy them by the dozen. And Mec-Gar are real pros and if they made these, they would work.