Green’s Dictionary of Slang

tin plate n.

a friend.

1924		G.H. Lawson Dict. of Aus. Words And Terms 🌐 TIN-PLATE — A mate.
1933		(con. WWI) A.E. Strong in Partridge Sl. To-day and Yesterday (1933) 287: Group. Where’s your Tin Plate? I suppose he has been making it too hot lately and they have kept him in the line.
1960		J. Franklyn Dict. of Rhy. Sl.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.
1992		R. Puxley Cockney Rabbit.