With Fowlers You see a lot of really "odd" sizes"early in the 18th century.Odd meaning by todays standards.
.69, .76,.71,.68,79,.70.59,78,77,67 are all listed in Neumanns book prior to 1775,
Its only till after 1775 do we see things starting to become smaller.There is lots of speculation as to why the bores started becoming smaller but I suspect the reason is that powder and lead was becoming more scarce as the war progressed and smaller bores meant that powder and balls would go a lot further with the smaller bore.
.67 .65,.62,.70,.65 ,.68 are just a few of the ones listed in the book post F@I war.