Hi Olaf:
I have since had an opportunity to check out the Shipbuilder edition of the Mauretania and can say with accuracy that the funnel tops had two transverse braces across their ellipses. (There is a picture of the funnels being assembled in the yard and in the foreground is a large metal ellipse with the two transverse braces.) Given the black-on-black nature of paint & soot and the fact that there are very few photos taken at an elevation above the funnel tops, they are very hard to notice - they may have rested slightly recessed in the funnel caps.
Another point about the funnel rigging is that all the funnel stays seem to have been anchored to the roof of the deckhouse along the outer edge - although there may have been a pair of stays that tied down to the bridge house. This differs from Lusitania where half of the funnel stays (namely, the upper ones) were anchored to the boat deck.
Cheers,
Mike M.