Almost looks like the handle was made from a piece of diamond plate and welded on.
That was the first thing I thought also, Could possibly be a real old piece that the handle broke off of and they replaced it with what they had laying around, It appears to be beyond any usable condition and if it does not have any markings in my opinion it is suitable for a planter outside and not much more, Is it actually cast iron or is it made from a piece of forged steel, The side walls look pretty thick,
For me it is not something I would invest much time or money in.
