This ad, from the February, 1923 issue of Radio News, clearly shows Atwater Kent's strategy for avoiding the Armstrong regeneration circuit patents (owned by Westinghouse by this time.) All of the sets were sold without the middle variometer and as such employed no RF amplification. At the bottom of the ad however, Kent placed an additional variometer. No mention is made of its purpose, but when installed by the purchaser it made the set regenerative, adding significant boost to the RF signal and thus the overall performance of the set.