After further research I've answered my question. The 1-2 Band Stop is a locating pin, inserted in the transmission housing, that keeps the 1-2 Band in the correct location so that the servo pin will properly engage the band. When it is not in place the band can become mislocated and cause sudden loss of all forward gears.
The 1-2 Band design was changed in early 1990 (January). Any 4T60 transmissions manufactured after this date will not have the pin (although the hole is still present in the housing).