This is one of my favourite agaves. I've had it for a few years now and it was sold as a suspected a. filifera cross, given it came out of a batch of a. filifera seedlings. There was no point in trying to find out more, so I just left it.
Having put some of the agaves out in the greenhouse (from the last post) I just happened to put one of my other hybrids in front of it. This one is a. victoriae reginae x a. obscura (also known as a. polyacantha var xalapensis)
I was struck by their similarities, the terminal spines, the shape of margin teeth, and the curve to the leaves. I also bought them from the same source, so it will be interesting to see if the smaller one continues to grow into a similar form.