Ive seen a number of people on FB forums linking to these supposed 420AH batteries, including some well informed posters. I always point out the fact that the physical size and weight of those fake 420AH cells are precisely the same as well known 280AH cells. Always check the size and weight as there is no way to create a battery with 50% more capacity in the same size and weight.
Of course, if its too good to be true, it always is not true, 100% of the time IMHO