These letters were penned in 5728/1968 after the Rebbe had announced Mivtza Tefillin, namely putting on tefillin with any Jewish male over 13 years old, anywhere (almost) and anytime (from the z'man in the morning till shkias haChamoh.) Rav Yitzchok Hutner zt"l, Rosh Yeshiva of Yeshivas Rabbeinu Chaim Berlin in Brooklyn writes to the Lubavitcher Rebbe, זצוקללה"ה regarding his concerns that he had over this new campaign, including the need to tell the people you were donning the tefillin on that there were parchments inside that had the parshios of krias shema written on them. We see the respect RYH has for the Rebbe, and how he wants to understand the Rebbe's words and opinion in the matter. There's also the denying of ever speaking out against Lubavitch during the '67 Agudah convention, or being present at that or any other convention for that matter, to the point where he begs forgiveness and asks to be blessed by the Rebbe.
No attacks and condemnations, only respectful dialogue
"ישנו יהודי וואס קאכט זיך אין מהר"ל..... יה"ר שזכותו של המהר"ל תעמוד לו להינצל ללא פגע"
הוד כבוד אדמו"ר מליובאוויטש שליט"א
ביקרא דאורייתא ...... יצחק הוטנר
How much more proof does one need?!