Front Master Cylinder Rebuild on Rear?

Just to clarify, I can't speak for that B2, but these are from a 1979 and I believe a 1980 or 81 A models if I am correct. My 1982 is exactly the same.
All are 5/8".




Side by side - rear and front pistons. The rubbers can clearly fit either, the only problem I'd have would be doing it without damage. But like I said, it has been done.

The groves in the front lower rubber - I haven't seen it in another master cylinder so that's definitely not an issue. If I remember correctly, the rebuild kits don't have them as well.

I just wanted to tell you that not everyone is the same.
but the same cylinder fits

Stonehill, do you do re-sleeving? You have loose sleeves in your photos.

By the way, yes, they are not all the same exact design. The 1979 (or was it 80?) rear master cylinder piston was a bit different design than the 1982. Not important, but one had that plastic piece (where the spring goes) and the other didn't. So yes, not all exactly the same, but those I've seen all were the same size inside (diameter). All I've seen were A models.

yes the inner part has been replaced with a new one.

Seeing as how we're on the subject of rear master cylinders, I've got a new one- assume, for a second, that somebody (me) decided to repaint his MC with caliper paint, and per the instructions cured it at 200 degrees F for an hour.

Now assume they (I) didn't realize that the access hole used to drill the two smaller holes into the bore was sealed by a plastic plug.

Assuming this happened, what would be the best way/material/whatever to seal this?