Hello, I am a new member of this Forum and in looking through older postings I came across the discussion about Athyrium filix-fimina spp. angustum, or as it is now, A. angustum. Living in Eastern Canada, this is one of our native ferns. I have many on my property. I would suggest that the posted images are none of them A. angustum! Probably someone who grows the European Lady Fern along with some of its cultivars sent in the spores. I know this post is too late for the discussion, but just in case the originator is still on the forum, thought I'd add my 2 cents worth.
I'll post a picture of an A. angustum frond soon when I have more time and figure out how to do it!