In this case the push rod had two incompatibilities. First and most importantly, the groove for the retaining clip in the end of the cylinder - the clip that holds in the semi-hemispherical washer, and therefore the push rod end ball itself - was too deep in the cylinder...the washer is just thick enough that you can't hold it in, and I suspect that if you could it might have been engaging the pistons a little too much any. So that was just a plain no-go.
Second, while the new cylinder is the exact same length as the old one so has the same amount of piston travel, etc., the old one sticks further into the firewall; the net of that situation is that the new MC sticks into the engine bay about 3/8" more than the old one. There's nothing you can do with shims because adding them on the other side of the firewall only makes the situation worse. So even if the original push rod's ball would have fit nicely into the socket using the standard washer, the push rod needed to be 3/8" longer. That's a pretty easy job for even a novice welder but the retaining clip made the attempt fruitless.
Luckily, unlike the old MC that only had a dished ball socket, the new one has the dish but also a cylindrical extension (a hole recessing deeper into the piston plunger). That enables the use of either a ball-end push rod, or a more-common option that's just a straight bar. My custom solution uses the latter approach that will require probably a total of ~3/4" longer rod than the original but won't require reuse of the original washer and still won't fall out.
Below are pictures of the original vs. new differences I've tried my best to explain in English!
I will follow up later with the custom push rod which I'm using to get the length correct before ordering manufactured parts for the ultimate solution. The new push rod's end will need to be milled down to something like 7mm to accommodate for lateral play, but that's a simple job I can do on the bench.
End of old MC piston - only ball end is possible:
End of new MC piston - ball end or straight rod end possible (see "hole" inside ball socket):
New MC requires longer push rod because old MC is recessed further into the cabin than new now:
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