1290

1290 is an alt-right dogwhistle, that doesn't mean anyone who uses the number is a member of the alt-right, it depends on the context. Though you should still be wary of people who use or have it in their username.

It's referring to the Edict of Expulsion by king Edward I of England. In which he advised to sheriffs of all counties to expel all jews.

Alice: How'd you recognise they're a neo-nazi?
Alex: They had 1290 in their username.
Alice: Oh, yeah, that makes sense.