Thanks Mark did a bit of research myself and although I cannot definetely say that the case is original from the images I have found the dial and movement may be.
I found this image http://imageshack.us/photo/my-images/51/heuerleonidas.jpg/ which shows the round pushers which to me on balance looks more like the kind of case which would have been used in the 60's.
While I dont doubt Leonidas used cases with the flat pushers they are more anachronistic of 50's chronos I believe and I will not be pursuing this one it just does not seem right

: The bridge does look original, although unusually not guilded, but
: the bar pushers seem anachronistic - were Leonidas really still
: making non-waterproof chronos by 1964? I should stress I don't
: know, I'm genuinely asking the question. The merger was in 1964,
: by the way, rather than the 1950s given in the eBay posting.

: Post merger, the companies were used as discrete business
: divisions, with Heuer using the traditionally jewelled movements
: and Leonidas using pin pallet ones. I guess there were still
: stocks of jewelled Leonidas watches that could have made it out
: with bridges marked with both company names but again I don't
: know offhand - I really should get more of a grasp on Leonidas,
: my next history article to write covers the 1960s so I'll need
: it for that!

: A lot of corrosion on the movement, looks like water has gotten in
: at some point - don't know why they stopped cleaning for just
: that small patch, but it looks like it was more widespread once
: than it is now.

: With all that in mind, I'd go with Craig's estimate of upper limit.