LATEST UPDATE
Simpson has been presenting an assortment of one-off brushes exclusively for sale on its Twitter page (see first post in thread for examples). So far as I know, these have not been available on the main site. They have been selling fast - "within half an hour".
From a twitter exchange, it looks like this is how the horn and wood handled brushes recently announced will be sold:
"Will the horn & wood handles mentioned only be available via twitter, as seems to be the case with recent LEs?"
"Probably yes, makes things easier from an administration perspective."
"And how do we get further details (prices etc.)/order/pay?"
"If you see something you like, email enquiries@progress-vulfix.com for pricing, availability & payment details."
So there you have it. If you are a Simpson devotee/brush collector, you'd best sign up to twitter - indeed, as one poster said, "joined twitter today for this reason."
Simpson's twitter address is: @AESimpson1919