A few years ago, it was established by postings on this list that the viscosity of endolymph (in the cochlear duct) is similar to that of water. On page 443 of "Experiments in Hearing" (1960), however, von Bekesy wrote: "When the gelatinous mass of the cochlear duct was removed from the basilar membrane with a fine brush the traveling waves were observed with a larger amplitude and less damping." On page 469, he writes about the "viscous contents" of the cochlear duct, and at the bottom of page 475, he mentions the "gelatinous mass in the cochlear duct", which was "pushed aside (as with a fine brush)". On page 467: "If Reissner's membrane was removed and the material within the cochlear duct was washed away, the elastic properties of the tectorial membrane could be investigated [...]".
So, since the "gelatinous mass" was obviously not the TM (as I suspected in the past), what was it?
Possible candidates: see N. B. Slepecki, "Structure of the Mammalian Cochlea", in the book "The Cochlea" (1996), Section 2.5.2, "Claudius Cells". They cover the BM on the scala-media side. The term "Claudius cells" does not occur in von Bekesy's book, I believe.