Thanks for the correction Les. Lusitania was indeed assigned to be an AMC (a decision not seen through in the end) and I had assumed the "Q" designation applied to these conversions as well as to the smaller vessels.

Pete, you're right that Mauritania was the sister ship. That and a subsequent, even larger Cunard Line ship, the Aquitania, both did get requisitioned and converted for government service. I believe both were used only for troop transport and hospital duty.