I think the pale blue (and sometimes pink) cards with the cream border of stripes and dots is the earliest style bearing the Beauclaire brand. The first mention of “Beauclaire” is for buckles at 7½ and 10½ pence each during August 1951.
“Normal” button cards replaced the square cards. The method of attachment is simpler: sewing in place with a couple of stitches rather than threading through a strip of card and taping in place. Perhaps the buckles came loose sometimes?
The above style buckle was reproduced in (probably) the mid-late 1970s on a Butterick branded card. Beutron had cross promotions with Butterick from 1970.
Around 1978 the script for ‘Butterick’ changed to all upper case lettering.