Oliver Cowdery initially wrote the rather awkward “in flesh” in the printer’s manuscript; then he inserted the definite article the. This article probably occurred in the original manuscript (which is no longer extant here). The supralinear insertion is written with slightly heavier ink flow; perhaps Oliver made the correction when he proofed 𝓟 against 𝓞.
Otherwise the Book of Mormon text almost always reads “in the flesh” (22 times). In fact, there are five other occurrences that refer to Christ “manifesting himself in the flesh” (like Enos 1:8, all five are in the small plates of Nephi); thus the support for the corrected reading in 𝓟 here in Enos 1:8 is very strong. On the other hand, there is only one occurrence of “in flesh” in the Book of Mormon text:
This reading is found in the printer’s manuscript; the original manuscript is not extant for the book of Mosiah. It is possible that the occurrence of “in flesh” in Mosiah 15:2 is an error for “in the flesh”, especially when we consider the two other uses of “in the flesh” with the verb dwell:
For further discussion of the one exceptional case of “in flesh”, see Mosiah 15:2.
Summary: Maintain in Enos 1:8 the phrase “in the flesh”, the corrected reading in 𝓟.