No problem. I was really pissed when I came back from my first range day with my new money pit. A night of research and carving with my pocket knife was all that it took.I have about 10 of the M revised Pmags. None of them will allow the bolt to remain open after the last shot. I haven't used a single Pmag yet that would hold the bolt open, kinda irritating. Thanks for the link on the mag mod Tackleberry.
Original pmags have black followers, M revision have gray followers. M rev mags come in windowed and non windowed both and have for years. There are also emags these work with European pattern weapons like the HK 416, and emags also work with standard ar15s. The mags that come with the ACR are M revision pmags. Avnwo, I don't know where you collected your information but you may want to check the source.
There are some m revision mags that don't lock back on the last round with the ACR ( this may also be the case with the old pmags, but not my experience). This can be confirmed with Justin at magpul. I believe he also posted some info about this on arfcom under the magpul section, but you may have to search the archive.