Hi Maurillio,
I also have a plant like this. I bought it at Southfield ( a British nursery) in 1998. It is identical to the photos you have posted, but iI have it listed as a hybrid. I think that it was actually labelled as M. vetula f, pink flowers. I think I added the hybrid bit, as I have not seen any plants with field numbers, let alone one for a pink flowered form.
If it is a naturally opccurring form, it could be that it was produced in cultivation, but as a natural sport rather than a hybrid.
It would be good to know more.
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Chris43, moderator