The modern catalogues make no mention of 1864 at all and nor does the virtual museum.

The text for 1860 does change from saying Edouard Heuer opened a watchmaking workshop in 1860 to saying he founded a watchmaking company. I wonder whether that is the difference - 1860 was the foundation of the workshop and maybe the company as such was only formally incorporated four years later perhaps? Though the 1860 date does seem to be attributed for the foundation of the company now too. I guess the older the better when you're a Swiss watchmaking company.

What does annoy me in the post-TAG brochures though, is the attribution of everything to TAG-Heuer rather than Heuer. With no mention of the time machine that would allow them to do that, which you would think would be most worthy of mention... ;)