M rated is, to my knowledge, a slightly lower rating than E rated, which is what NC-17 usually translates to. Both AO3 and ff.net use the M(mature) and E(explicit) rating (although ff doesn't actually allow E rated content) but some people on livejournal and other such sites use nc-17 instead of E. Hope this helps!

In this instance, I AM counting Teddy as part of next gen :) Go wild!