http://www.keymancollectibles.com/exhibitdatechart.htm
This chart is pretty good.
1947 'MADE IN USA' 1/2"
1948 'MADE IN USA' 5/8"
1949 'AN EXHIBIT CARD' on left 'MADE IN USA' ON RIGHT
1950 'MADE IN USA' 7/16"
1951-1953 'Made In U.S.A.' (upper and lower case)
1954-1956 'PRINTED IN U.S.A' Printed script name on left
We also had another thread on this.