Here is a  comment from  The Christian’s Friend : 1896

https://www.stempublishing.com/magazines/cf/1896/The-Songs-of-Degrees.html

“All are grouped,” says a pious expositor, “around Psalm 127, which was written by Solomon.  On both sides there stands a heptad (i.e., seven) of pilgrim songs, consisting of two Psalms written by David, and five others, which have no name attached. Both sevens are divided into four and three …”

Does anyone know who the “pious expositor” is?

Wikipedia (referencing  The Flow of the Psalms, O. Palmer Robertson, P&R Publishing, 2015,  pp. 232–233) adds, “A chiastic structure is seen by many in these Psalms with Psalm 127 a Psalm of Solomon as center. Preceded and succeeded by seven Psalms of ascent, each side adorned with 24 occurrences of ‘Yahweh,’ a numerical symmetry evoking divine significance.”