I have all three, plus a glossy 'slick' meant to be pasted to a machine header card. My best info is that the set was issued in green first, then sepia, then black and white, so the greenies are the earliest.
As far as I know the green Exhibit is the first American Clay card issue. However, I still believe that the Hemmets is the rookie card and the 1962 Rekord probably predated the green Exhibit as well. I'd guesstimate that the Exhibit was followed by the Chocolat Simon and MacRobertson in 1964, then a wave from Europe in 1965 (Peretti, Lampo, orange background Swedish chocolate, Dutch Serie MLH). Also the Argentine 'pog' Clay which PSA dates as 1962 but which has a photo taken on June 18, 1963 (PSA wrong again...I am shocked), so not. 1966 we have the first Panini Clay and also the Mundo de Fantasia from Argentina.
BTW, can you see the difference between these two cards?
Both are blank backed. They look the same to me.