I have come across this infomation and thought it might be useful to others as it was to me.

When you have a postcard with a postage stamp but can't quite make out the year in the cancelled  postmark it may help with this guide.

1872--1 cent stamp

1917--2 cent stamp

1919--1 cent stamp

1925--2 cent stamp

1928--1 cent stamp

1952--2 cent stamp

1963--4 cent stamp

1968--5 cent stamp

1970--6 cent stamp

1973--7 cent stamp

1975--9 cent stamp

1976--10 cent stamp

1978--12 cent stamp

1981--13 and 14 cent stamp

1985--15 cent stamp

1988--19 cent stamp

1991--20 cent stamp

1995--21 cent stamp

2001-2002--23 cent stamp

I hope this will help when finding those treasured postcards.