[י"ב אדר תשי"ג]
...בענין מערין חיין - יעויין ברה"נ פ"ו סי"א: שאין רגילין רוב כו'. ונסמן בש' הציון למשנ"ב סר"ה סק"ג.
12 Adar 5713.
On the matter of Ma'ariv Chayin—see Reishis HaNefesh chapter 6, section 11: that most are not accustomed, etc. It is also referenced in the citation to Mishnah Berurah section 205, sub-section 3.
Summary
This letter clarifies that the custom of Ma'ariv Chayin is not widely practiced, as noted in halachic sources.