Is the overspin putter legal

No it is not a putter must be upright at least .5 of a degree and not have a multi level face

Straight from the USGA rule book:

A club is an implement designed to be used for striking the ball and generally comes in three forms: woods, irons and putters distinguished by shape and intended use. A putter is a club with a loft not exceeding ten degrees designed primarily for use on the putting green.