I don't understand the question about a 1/4" to 3/8" adaptor???
The Rycote comes as standard with a female 3/8" standard thread - 1/4" doesn't come into it at all.
I think Rycote supplies a 3/8" to 5.8" adaptor as standard so you can fit it to stands with that horrible US thread (horrible because the 5/8" thread is so fine that it's easy to stick and bind).
I might be misunderstanding Rycote's specs, but as I read it they are saying:
"Male thread: 1/4" Whitworth
Female thread (brass): 3/8" UNC"
In other words, I am assuming that the male screw on top of the Rycote is 1/4" and the bottom/female is 3/8". So it will certainly fit on top of a standard stand with a 3/8" male thread, but the T-bar that needs to fit onto the Rycote has a 3/8" female thread, while the Rycote apparently has a 1/4" male thread. Hence my question about fitting a bar onto it with a 3/8" female attachment.
My DR-100mkii has a female 1/4" thread on the bottom, and since this Rycote is designed to fit a recorder like that on top of, for example, a boom pole, it makes sense that the male screw on top of the Rycote is 1/4".
Regards, Christine