Players who are eligible to transmog this appearance need to have achieved Knight-Captain// Legionnaire back in vanilla or 1800 RBG rating. They can purchase transmogrifiable versions from vendors in Area 52.
From thew top comment here… https://www.wowhead.com/transmog-set=849/field-marshals-battlegear#comments
I’m assuming here, but there would usually be 50 people telling them that they are wrong, if they were incorrect.