Thank you all for your interest/help with my potential rootstock. It has been quite a learning experience for me - very much helped by forum members. If you Google citrus rootstock, several websites come up with dozens of different rootstock varieties, none of which closely match my tree. I am in an area where citrus cultivation predates introduction to the current US and there seem to be several local varieties of "lemon" that have been around so long that many consider them naturalized here. Some are almost as big as a football. The fruit from my rootstock was recognized by locals as one such variety. It looks very similar to the "brain" citron but doesn't have the characteristic thick albedo of the citron so I assume it is some kind of hybrid that originated in earlier years of citrus culture here. The rough lemon looks fairly close Laaz and descriptions note a lot of variation in the plants, so that may be it.
It tasted somewhat like lemon but was much sweeter with a slightly bitter aftertaste. I will try to find out more and let you know what I find.