House of Barrington or Sugg Sports I'd guess. 1975-76 was the first season that replica shirts were available in great numbers for people to buy, prior to that you could get a red and white striped shirt or even a white shirt with a red collar but they were generic cotton things that could be Sunderland, Exeter, Brentford or Liverpool (white shirt). Admiral were the first to start to produce 'designed' kits and mass produce them. Nowadays they'd slaughtered for having too many 'template' designs, but at the time this was new and unique.
Bit of irony in the only teams that got the chevron design shirt was us and West Ham
http://www.truecoloursfootballkits.com/articles/the-kit-cupboard-special-admiral-in-the-70s