Asked this on European Palm Society and Paul Spracklin generously took the time to say :
"It is definitely NOT any form of salmiana, ferox or otherwise. I'm fairly sure it isn't any form of americana. It might be a form of parryi/havardiana but those slanted back teeth are not usually seen in that group . More likely a less toothy form of potatorum. But hard to say with any certainty."
Since then it seems to have grown at a fairly decent pace and has created a few pups, but I've no idea how large it will get, nor if it is hardy. I'm suspecting it might be quite a small agave due to it pupping already.
Anyway, here is a more recent pic: