Hi Matt,
As an add on to what Eric has just posted a March 1912 Fares Rate and Deck Plan booklet indicates a slightly different shape to the Observation Room on the Lusitania to the one in the Shipbuilder; while most of the Observation Room on the Mauretania had been turned into 2 bathrooms and 2 linen rooms, with rooms B-5 and B-6 being given portholes.
It would also seem that the Lusitania's Observation Room had far fewer portholes than is depicted in the photograph in the Shipbuilder. There only seem to be 8 in total, whereas the Mauretania had 7 in what was left of her Observation Room.
The photograph in the Shipbuilder also shows an raised area under the windows.
I hope that helps,
Lester