It is the "Referencia" field. In my memory it matches on that field.
Probably uploading through combinations will not add but over write the stock posistion.
I have made a customisation to the upload controller not sure anymore what excactly. I perhaps it concerned the reference field. I attached it. Put it in you <docRoot>/override/controllers/admin/ directory and test. I have no time to have a look in depth, perhaps this weekend. If stocks are overwritten it should be easy to adapt this in controller.
Note this is not the earlier mentioned stock syncing code.
Rg,
Leo