Finally had a chance to dig around a bit on this.
Tannoy themselves don't describe them as gaskets, the phrase they use is "....trim rings ... to smooth out the otherwise discontinuous contour to the HF wavefront" - in other words, they are there to deal with diffraction.
Page 14, Section 5 of the Tannoy Monitor Series service manual - this is a later series (NFM and DMT) but the design is common going all the way back http://warehousesound.com/r/tannoyMONITORmanual.pdf
Indeed if you look you can still see a version of these rings (without holes for screws) on the current Legacy series.