Basic rules: (correct me if I'm wrong)
All years are approx.
1900-1920 Wooden box with open keyboard.
1920's 1930's- wooden box with small buttons, some bare wood, some painted crazy colours
1930's - start of plastic covered cabinet. Large logo either in writing or inlaid in stones. Small buttons.
1940's early 50's- famous greys. Small buttons, the well known small logo, and aluminium grill
mid 50's - about 1960 - red boxes with blue badge
1960's - red boxes with red badge and small lettering "paolo soprani"
1970's - larger lettering of "paolo soprani"
1980's - the end of the "real" paolos, large italic lettering, after mid 80's the boxes were made by bafetti, same name, different instrument.
2000's - jubilee came along.
A picture would help a lot. The above are estimates - some boxes vary from the mainstream production of the time.