The older 120R did not have a pressure tap and ran well without it. Later ones came with a tap in the muffler and seemed to hold their settings better. I installed one on my 120R's muffler and noticed it maintained a more constant run through the pattern. I also note that the 2 1.55R's I recently purchased have pressure taps installed on the mufflers. On your 120R, no, not absolutely necessary, but it definitely could help.
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Phil