can anyone explain the different versions of 65/66 glidrose/Philadelphia sets different backs different copyright dates any help would be apprecated thanks

I'm not sure myself, I have just completed the 65 version, I can say all the cards i have are printed with 1965 on the back.. I am chasing the 1966 verson but do find the ads confusing.. everytime I see a set adverstised as '66 the cards look the same as my '65 set..