The 3rd Class cabins were numbered 1 through to 260 (I'm going by what Lester said as I have no plans to refer to at the moment). To avoid confusion, none of the 3rd Class cabin numbers repeated*, the only way to indicate where to find them was designated by a section letter.
*The only staterooms to repeat the number were 9 and 9A and the alternate accommodation staterooms aft on G Deck, G 1 through G 41.
Brian - the inside staterooms did not have sidelights; it was the outside rooms that had sidelights.
Thomas - I'm sure once upon a time there was a master list which recorded where each of the 3rd Class passengers were placed. There was most likely only one copy of this list which went down with the ship.
The only other way to have a record is from the green immigration cards which were issued to the passengers. They were required to keep these cards on their person at all times and on the day of arrival into US to have them pinned to their clothes and in plain view at all times. I'm sure that almost all, if not all the 3rd Class passengers actually escaped with their immigration card. Various 3rd Class passengers' bodies were recovered still with the card (which is how they were identified). Given this, I'm sure there were a few hundred of these cards that survived the sinking ... the only problem is that they did not seem to survive the ravages of time as only a handful seem to have surfaced.
I would guess, Thomas, that your grandfather and great-grandmother also escaped with theirs. Perhaps you may know if it still exists or did at one stage?
As Lester pointed out, generally, families were berthed in sections K and M. I cannot confirm this, but I think that fathers and sons over the age of 16 were berthed in the forward sections, so I'm not sure if you great-grandfather was in the same cabin as the rest of his family or if he was berthed with other passengers in the forward section.
Best Regards,
Daniel.