confusion with Birmingham marks

[geoff956]

As a relative newcomer to silver collecting I often get tied up with Birmingham date letter marks. E.G. letter 'O' 1913 or 1938, letter 'S' 1917 or 1942, letter'V' 1920 or 1945.
I realise of course that the earlier ones are lower case, but when seen in isolation it's difficult to tell which is which.
Any advice much appreciated - thank you.