Good morning Taylor,
Thank you for your recent question concerning the "in" lower groupsign in "1-in." for the abbreviation inches. The UEB Literary and Technical committees agree with your claim that the "in" lower groupsign may be used based on the references you cited.
§10.6.8 and §10.6.10 would apply in this example. The hyphen terminates grade 1 mode when set by the numeric indicator. As a result, the "in" groupsign followed by the period should be used. Both are in the lower cell but connected through the hyphen in the sequence to the upper dot 4 of the numeric indicator.
Dan Gergen, Chair
UEB Literary Committee