Some sort of quince? (They come in different shapes.) Or if not, a quince relative?

Chaenomeles? Can we see the outside of the fruit?

this time of year ... and yellow..

and if tasted.. bitter as heck .. and also hard as a rock ...

probably one of the quince mentioned ;..

ken

iBarok, here's the short version: there are two types of quinces: the quinces grown for their fruit (Cydonia oblonga) and the fruit of the flowering quince (a few different Chaenomeles species).

Both types of quince are edible, though the Cydonia quince must be cooked, and the fruit of the flowering quince must be processed properly and is only used for preserves and liqueur.

There is also Pseudocydonia sinensis, Chinese Quince.

Here is a link that might be useful: Pseudocydonia

You can add a few sliced Chaenomeles fruit to an apple pie. They impart a specific aroma.