Blue Star Line house flag 23mm gilt button

 Blue Star Line house flag button — the swallow-tailed burgee with a five-pointed star, roped border, marked “BLUE STAR LINE LTD”. Blue Star Line was a British shipping company (founded 1911, part of the Vestey Group) known for refrigerated cargo ships running meat from South America and Australia/NZ to the UK, plus some passenger routes.

The backmark is “OLIVER & Co, LONDON” — one of the London button/badge makers who supplied Merchant Navy uniform buttons. At 23mm, that’s a larger “coat” size button (jacket/overcoat), as opposed to the smaller 15–19mm sizes used on cuffs or caps.