Rohrquint[e] German
Roerquint Dutch
Rohrflötenquinte German
Rohrflötquint German
Ausdley and Irwin define the German names as denoting a Rohrflöte at 5-1/3' pitch, though Osiris contains as many examples at 2-2/3' as at 5-1/3', as well as nearly as many examples at 10-2/3'. The name Rohrquinte has also been used as a synonym for Rohrnasat. This is a very old stop; the earliest known example dates from 1646.
It could be this stop, rounding 5 1/3' up to 6' . Note the stop it's made from also has a prominent twelfth.
Definition courtesy of http://www.organstops.org/MainFrameN.html
James Pressler