It definitely looks to be homemade or fabricated- looks just like a radiator valve key or a clock winding key!
The fact that the metal at the ‘business’ end is so thin suggests it doesn’t have to apply a lot of torque to whatever it operates- it’d just distort and split open if it did.
The end has just been squashed cold to form the triangle like you said- it has the tell-tale crease lines down the centre of each ‘flat’ of the triangle. Normally a manufactured key would be cut or broached into more solid material to get the shaped section up the end- look at an S&G or LaGard timelock key for example.
It wouldn’t be very strong as it is, so can only guess if it was for any sort of simple lock it may have been homemade as window lock key or something like that?
