Yes, I realize the one I'm looking for is not the oldest
megaminx out there. But unfortunately it looks like everyone that used to sell the PVC Megaminxes (The white one at the bottom of your write up, I believe) now sells a "new improved" version that they either call "II" or "Generation II" or whatever. However, the pictures on the listings themselves are of the old versions, and they're sending out the new version instead.
Here's a comparison, the "new" on the left and the "old" on the right
IMG_5951.JPG [ 789.38 KiB | Viewed 998 times ]
The new version is hollow, made of cheaper plastic, and lacks that grooved "foot" on the inside of the mechanism. What's interesting is that I tried substituting pieces from each puzzle and they both fit exactly.
The only reason I'm looking for the solid minx is for modding purposes. The solid plastic makes things much easier instead of having to waste other materials to fill/cap, and I just think the plastic itself is sturdier.