US2772427A - Book backbone structure with concealed
In efiect, the head 17A of the rivet is gripped between the two bonded paperboard backbone pieces, and the radial grooves 16A (Fig. 3A and Fig. 5 adjoining the rivet holes serves as a concealed, bulkless reinforcing means to augment the bond between the backbone pieces themselves in this region and between the rivet head and the inner backbone ...
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