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.