Yes I have found they seldom come up on eBay, apart from the odd gem like that Plisson on Olive wood the other day and the one you posted the brush pickings are thin on the ground.
FWIW I recently purchased a GT-3 from their Curzon St shop. It says Best on the handle, cost GBP 85.00 has a knot of 24.5mm & a max loft of 47mm ( i know this cos I just measured it with my trusty very near caliper ) I believe this brush is a renamed Simpson Duke 3, the GT-1, 2 = Duke 1 & 2.
I have a feeling that this is Simpsons super badger hair. What ever hair it is it is a fantastic brush.