Just to be clear, when the stuff finally hits the fan hard here in MA and even pre-ban hi-caps become illegal with a "life prison sentence for possession" (same as for bump stocks), I will have no choice but to go with Mec-Gar 5900-series 10-round magazines for my 9mm full-size double-stack 3rd Gens. I have a small number of S&W OEM 5900-series 10-rounders, but not nearly enough if 10-rounds is to become the only legal option.

I'm actually very glad that Mec-Gar is still making them.
My bigger problem will be with my 6900-series pistols... and that's where I say that it makes no sense (to moi) to go with full-size 10-rounders in compact pistols regardless of magazine brand... hence, my continuing search for 6900-series 10-round magazines. Except for the Robertson cut downs, that means finding more S&W OEM 6900-series 10-rounders regardless of cost.
The Robertson cut downs probably work fine, but I'm going to stick with S&W OEMs if I can find enough of them.