What is the meaning of No. 1 mess kit?

[Beck's-a-kilted]

Thank you to everyone. The abundance of information is exactly why I had confidence in asking the rabble. It is true that a better answer could be given if the question led one to a particular region/country. To pinpoint this origin - the events will take place on the Isle of Mull, Scotland. With the information given from everyone, and myself having no military background, I should think my doublet or PC will be fitting.

[Dixiecat]

I presume that Beck's-a-kilted is from the UK or a commonwealth country because the invitation used typically UK-speak ("dinner jacket" being the big tip off)

The big tip off is 'doublet'. But then, maybe because I'm Canadian, 'dinner jacket' is normal?