Over a decade, Tyro. If the shell "Lifter" has a slot in it, you have it. I'm sure both of yours are OK.
BTW, my Bird 870 doesn't have Flextab, but it's a parts gun and not for serious stuff anyway. I will say that even when stuffing in hulls during hotnfast dove deluges,I've not had any problems, but I load pretty forcefully anyways....