I'll repeat, but more distinctly.
Canadian postage stamps went "metric" in September 1967 (the first stamp to use metric sizes was the 5¢ commemorative issued Sept 15, 1967 honouring Governor-General Vanier [Scott 474]).
All but three of the Centennial designs (6¢, 7¢, 8¢ Library) were first released February 8, 1967 using imperial measurements.
The 6¢ Centennial stamp was the first definitive
issued under the metric standard for stamp size introduced late in 1967.
The Centennial Study Group newsletters (part of BNAPS) and my 434-page book, 1967-1973 Centennial Definitives
, discuss these details.
See attached PDF which discusses the Centennial stamp sizes, presented with my (author) permission: www.adminware.ca/newsgroups/...tampSizes.pdf