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http://www.bricklink.com/v2/catalog/catalogitem.page?P=3626a#T=CCitation:
3626a - Minifig, Head (Plain) - Solid Stud - Item Info :
Years Released: 1975 - 1987
Weight: 0.57g
Size: 1.26 x 1.26 x 1 in studs
Citation:
3626b - Minifig, Head (Plain) - Blocked Open Stud - Item Info :
Years Released: 1990 - 2016
Weight: 0.54g
Size: 1.26 x 1.26 x 1 in studs
Citation:
3626c - Minifig, Head (Plain) - Stud Recessed - Item Info :
Years Released: 2009 - 2016
Weight: 0.54g
Size: 1.26 x 1.26 x 1 in studs