I think the M designation has previously been used for maritime ready Glocks for LE/mil customers in some countries.
My suspicion is that any major modifications to the Glock design will be the result of the XM17/MHS competition and not the FBI solicitation. Glock need only remove the finger grooves from the frame and offer a slightly redesigned magazine baseplate to meet the FBI requirements. I'm doubtful that either would prompt a 'generational' change or a new armorer's manual. However, the XM17/MHS requirement for a manual safety would certainly require such a document.
"When the phone rang, Parker was in the garage, killing a man."
While not required, to be competitive the Glock would need ambi slide lock levers. I think that could change the design substantially enough...
Not knowing what's cooking, they could also be killing two birds with one stone and have a modular FCG with removable safety ala HK USP or the M&P.
"Are you ready? Okay. Let's roll."- Last words of Todd Beamer
I would be a little surprised if Glock embraced a manual safety with open arms like that... but I suppose we'll see.
Some genius in the local police bought some glocks WITH a manual safety, the following orders were normal glocks. The safety is not very ergonomic IMO.
I don't have a picture, but they look something like this: