Interesting thread! There certainly are a lot of case options out there now; there were many fewer when the oyang 3646 and 6154 cases were created (about 8-9 years ago now).
When the 3646 case was designed, there were no ready sources for gen construction style 3646 cases (bezel and back screwdown onto movement holder sandwiching the midcase). The most common case was the Silix, which was not gen design. Nightwatch was doing great custom jobs where he would core out a Silix midcase (I believe), and then add a custom movement holder, bezel, and caseback to achieve the gen construction. He would also redo the lugs as soldered lugs in thicker gen style than the Silix lugs. The oyang 3646 was an attempt to recreate gen construction. It was crude to start and required a lot of custom reshaping, as well as soldering or welding of lugs after bending them from straight wire.
A few years earlier, a guy had offered gen construction cases for about $5k each and sold a few. That ended in some controversy over cases never delivered and money never refunded, apparently; some of the mods might know that ancient history from about 15 years ago.
This case and the 6154 were both machined from 316L steel by a shop working off CAD files using a 5-axis mill.
The 6154 was a much more ambitious case. Again, at that time, there were no cases that were anywhere close in terms of gen construction. The main elements in the gen that needed to be replicated were the two-piece bezel, and the two-step dial projecting up inside the inner bezel. This latter feature was what allowed the midcase to have a very slim profile like the gen while holding the relatively thick 616/618 movement and a sandwich dial. The "References" book gave a measurement of the midcase height of 5.7mm, and available cases (Silix and Athaya) were 1-2mm thicker. The height of a 616/618 movement is about 4.7mm, so incorporating the movement and a 1.5-2mm sandwich dial required this design. I think the oyang 6154 case was the first to do this, perhaps the first to notice this feature from studying photos of the 6154 and incorporate it into a homage case. The other feature was a magnetic shield inside. So, much of the fun of the 6154 case is its innards and thin profile. Looks like other case designs may have caught up, but the oyang 6154 was probably the earliest gen design case available, and was literally built around the 616/618 movement like the original gen. If you look at the design, every fraction of a mm of thickness is used; there is no empty space in that case when the movement and dial are in place. Another feature it had was the thin domed caseback, while other cases at that time had thick casebacks like the 6152.
Here are some pictures of the oyang 6154 design...
CAD layout of all assembled parts in wireframe:
CAD display of all parts, from top-down the crystal, outer bezel, inner bezel, two step sandwich dial, movement holder, midcase and crown, magnetic shield, and caseback:
Comparison to the Silix 6154 case available at that time:
Comparison to the Athaya 6154 case available at that time:
Comparison to a picture of a gen 6154 profile:
"Legendary" is an overstatement, but these were labors of love from someone who really wanted 3646 and 6154 gen-like construction cases at a time when they weren't available.
