I'd be interested in hearing how you uncover such specific dates as well, Aline, but I can offer two thoughts from someone who's spent a lot of time in on-line archives from Romanian Jewry of that time.
First, many marriages were recorded well after the fact. In the eyes of the authorities, that made most children "illegitimate." I have several examples of marriages taking place when the children were already in their teens.
Second, if you are trying to distinguish between two possibilities for your great-grandmother's father's name, I think "Grunberg" is more likely. "Osias" -- often "Osias Hersch" as I've seen it -- was a common first name. By the same token, I have come across a number of Grunberg surnames from the period (and am even distantly related to some). I suspect it was a matter of a confused form, and it's likely that your g-g-grandfather on that line would have been Osias Grunberg, father of Ghitl.
I'm happy to be corrected by someone who knows more than I do, but that's my educated guess.
Good luck
--
Joe Kraus
Shavertown, PA
krausj2@...