W124 ASD is basically same(discs between friction ones are different) as W201 16V LSD - I have many differentials in my garage. 5 friction discs both sides, W126 LSD diff is absolutely same but bigger one.
They are basically the same design, but the 190E-16 LSD is 185mm, while the 500E (and 500SEC / 560SEC) is 210mm. Totally different units. You can only use the 190E-16 with the 185mm diff on W124's with either M103 or diesel engines. Also, the 190E-16 LSD unit only works with 3.07 and 3.27 gearsets... it won't fit with the more common 2.65 or 2.87 gears in 185mm diffs. Check out my spreadsheet here
I just opened my 500SEC differential to make there better locking mechanism. I have done a lot of these for W201/W124 ASD/LSD. Yesterday I did it for some 500SE and it has open differential - it`s not the best result but it`s better than one wheel drive.
The spring blocks do work, but it's not without drawbacks. In addition to not being able to use the factory C-lclips to retain the axle flanges, the blocks also wear against the gears, leaving shavings in the gear oil. See the photos below. This might be ok on a race car where the lube is changed frequently, but I don't think I'd want it on a street car.
I think 1-gen straight-6 W126 use same size differential housing as W124. Basically W124 ASD/LSD could be mounted inside W126 housing.
The W126 and W124 with 210mm housings basically use the same carriers, so yes, the LSD/ASD units are interchangeable, but ONLY for the proper gear ratios. You cannot use the 2.47 LSD from a W126 in a W124/R129 with 2.65 or 2.82 gears, and vice-versa. The carriers have different dimensions. It may work if you swap the entire ring & pinion, but that's absolutley not worth the effort. Get the housing needed for your vehicle, with the gear ratio you want, and then obtain the correct LSD carrier for it - don't try to cobble the wrong LSD carrier in there with spacers!
FYI, I've built an LSD for my W124 300D (with 185mm diff) and also for my 500E (with 210mm diff). Photos are here