Doesn't sound like an estate to me, just another example of a trust meant to simplify administration and still open three years later.  I don't know how IRS will react if you file as a trust.  The EIN might be in a series reserved for estates (did they keep a copy of the application?).  I can't think of much difference it makes, except in the amount of the exemption if all the income has not been currently distributed.  The advantage to calling it an estate would have been choice of fiscal year, but you are long past that.