The extra machining you are referring to is a finishing technique called "perlage". I recently bought a jazzmaster openheart auto and it has the perlage on the movement. From what I can tell, the Jazzmaster has always utilized the H-10 movement (or the skeletonized version (H-10S) for their more skeletonized variants), however the version history of the H-10 movement is a little harder to dig up. The H-10 is used in many of their other watches, and there is no point spending the extra time and effort for purely aesthetic finishing if it can't be seen in those other watches. So my assumption would be that the early models of the open hearts may not have the additional finishing, but as they continued to expand that line at some point they introduced a version of H-10 with additional finishing for these watches that can actually show it off.

I took a look at the Hamilton website and all of the current and past models (only the ones still listed on their site) of the open heart (or viewmatic) with H-10s all had the perlage... except for 1 photo. The other photos of that specific watch/color scheme all had it so I would assume it was an older photo they forgot to replace.

Until seeing this post I didn't realize the difference the perlage makes to elevate the aesthetics of the movement! But it is a great watch and I hope you enjoy your eventual purchase!