I've found a very interesting photo on the website http://www.northatlanticrun.com:
It shows four photographs of an 1998 auction catalouge. The interesting one is the one on the upper right side: It shows an very elegant room which is called "1st class lounge" on the RMS Mauretania. But fact is: This is definitely NOT the ship's 1st class lounge (for comparison: photos no. 5 & 7 on that site: http://www.merchantnavyofficers.com/cunard6.html).
But what room is it? There is no equivalent to it on the RMS Lusitania, it looks like a gallery room or something like that. Were there any rooms added to the Mauretania during her career? Any ideas?
Best regards, Ludwig