My MMs died in infancy this year but I noticed that the F1 cross with German Queen has noticeably exerted stigmas. Is this a MM trait and also is MM a pimpinellifolium?
You'll find different opinions about MM and whether it's a currant or not, please see the link below.
The origin of the seed for MM was Craig LeHoullier and he says it's pea sized, but that alone doesn't make it a pimpinellifolium.
In the SSE YEarbooks it's in the regular sections, not the other species section. But then not everyone can tell the difference re foliage, etc., so that's that.
I've linked to a general Google search below and you'll read all sorts of opinions.
I've not grown it so I can't say anything about it.
Perhaps it's like Matt's Wild, which is a cerasiforme, a half domesticated cherry tomato with a retracted stigma. Perhaps it's a stable interspecies cross as is Sara's Galapagos.
It used to be said that the currants had an exherted stigma thus the pollination by insects, but it's now known that only about half of them do, so looking at that trait doesn't say one way or another what it might be.
I don't know how helpful this is, but whatever.
Here is a link that might be useful: Mexico Midget