Hi Glen,
Sadly not in the pile of literature and files I have, but it is mentioned a couple of times in the "manufacturing booking in and out bibles" of which there are two. It refers to rounds and barrels.
It refers to several stainless steel barrels fitted to M85's for trials as requested by ITDU Warminster as well as a 300 WIN as a consideration. All were returned to PH and there are no further comments as to why they were built and Bill can't remember why either. Special projects for special tasks, Peter L may have an idea!