In the printer’s manuscript, Oliver Cowdery initially wrote “the tower Sherrizah”. Later, probably while proofing against the original manuscript, he supralinearly inserted the preposition of (the level of ink flow for the correction is heavier).
Sherrizah is apparently the name of a place (probably a city, but also possibly a land—or perhaps both, a characteristic of Nephite naming):
In other words, Mormon is referring here in verse 7 to the tower in the city or land of Sherrizah rather than to a tower named Sherrizah. The of helps facilitate this reading.
We have already seen numerous cases of “city (of ) X” and “land (of ) Y”, with some variation regarding the preposition of (see, for instance, under 1 Nephi 11:13 for “city (of ) Jerusalem” and under 1 Nephi 17:7 for “land (of ) Bountiful”). It has become clear that it is best to let the earliest textual sources determine in each case whether the preposition of is there or not for the phrases “city (of ) X” and “land (of ) Y”. A third case that is similar involves the phrase “hill (of ) Z”. As explained under Alma 2:15, most instances of this phrase lack the of, but once again variation is possible: there is one instance with the of (“the hill of Shim” in Ether 9:3) and one without (“the hill Shim” in Mormon 4:23). Thus variation seems to be inherent for these three phrases. But “the tower of Sherrizah” appears to be different in that Sherrizah is not the name of the tower, but the place where it is located. The critical text will therefore maintain the of here in Moroni 9:7.
Summary: Retain in Moroni 9:7 the of in “the tower of Sherrizah”, the corrected reading in 𝓟; in this case, Sherrizah is a place-name, not the name of the tower itself.