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.