Here's something interesting -- a full sheet of 300 S-6 Savings Stamps of 1958. Except
they are monocolor, with no red flag and minuteman! Actually all
sheets I found were like this, including the 1961 series, with the 50-star flag. I wonder why they didn't proof the bi-color stamps. I may eventually come across those.
And another observation -- my 2016 Scott Specialized lists all these stamps as being printed in sheets of 400-subjects, with 4 panes. Clearly that was not the case for all of them.
Addendum: The 6-pane, 300-subject sheets were used to make booklets of 10.