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I think it is a lot more than a guess. Not only is VW telling us (with the fitment guide) that they are the same, we also have all this photographic evidence of actual MC's pulled from cars made in 2015-2019. Unless you think that is all somehow fake?
I do agree it is a bit confusing with all the different models and part numbers, as to which ones have the 23.8 mm piston like the one you bought. You never answered, do you no for sure what car that came out of? Oh unless, is that outer lip I was referencing on yours something other than 46 mm? Seems highly unlikely, but that would certainly throw off the photo measurements if that was the case.
More confirmation never hurts, but I think at this point we have more than enough information to say with a high degree of confidence that all the MK7 GTI's and Golf R's already have a 25.4 mm MC piston, no?