'59 Salesman Samples are legit and were issued as 3 card panels. The card on the back does not correspond to any on the front.
I've got a couple, but not one with Mays on it. This is the front and back of one of mine, for some reason I scanned them at different sizes
Salesman Samples were issued by Topps from '52 thru '67, all 3 card panels with the exception of '60, which has 8 cards. The '55 panels also include a '55 Doubleheader on the back, as seen below.
Bowman issued them in '54 and '55. The '54's are 4 card panels, 2x2, with a sticker on the back. The '55's are 3 card panels with a sticker covering the entire back panel, as seen below
Fleer also issued a 3 card Salesman Sample in '59. I have not seen one from any other years. I have a whole bunch of 3 & 4 card panels from other Topps, Bowman and Fleer issues, but without any ad copy on the back they just seem like strips of uncut cards, and not ad panels.