Thanks for the reply, Trek did come back to me and confirmed the bolt to use is W547868!
It seems both part numbers can be used, depends which copy of the diagram you have?
I'm sure this may have been covered elsewhere but I can't find the answer.
I have a Rail 5 Gen 2, I'd like to fit the Rockshox Super Deluxe Ultimate Thru-Shaft from a Rail 9 Gen 2, the problem I have, is on the Trek website on two different documents they list two different part numbers for this...