All I can say is that the Slough link implies it was the first Mars bar. And it looks plausible to me: it feels more likely that Forrest Mars Sr. would use his name for the UK Mars Bar copy of the original Milky Way and then his parents would take the name back but had to use it for something else in the US; the alternative would be that Frank and Ethel Mars created both the Milky Way and Mars bars in the US and then Forrest chose the wrong name for his UK product.