 This additional pay, in addition to the number of specialists of the sixth and seventh grades in relation to the total number of enlisted men of these grades, was defined in the National Defense Act of 1920.While the official insignia was a single chevron, it was not uncommon for local commanders to authorize local use of specialist insignia which consisted of one chevron and one to six rockers depending on the pay grade of the specialist (one rocker in the 6th grade, six rockers in the 1st grade).To indicate their specialty, a trade badge was sometimes inset between the chevron and the first rocker.