The MacManus link seems correct only because the advertising "MacManus John & Adams" is also spelled as "McManus John & Adams Ad Agency". This company was in Michigan and was Pontiac's ad agency.
https://www.hotrod.com/articles/1982...-confidential/
Also
https://digital.library.in.gov/Record/IUPUI_IMS-4121
There are many other text articles that use "McManus" however I can't find any images showing that company name. Also same with "MacManus" ... this might be a case for some collectors or researchers in that hobby, although I doubt it will identify where this shirt was used.