The shell bone script of At or Similar Prepositions is
The left top is mouth (see Post 152), the left outside is the bamboo tubes or other materials, inside is the music wind tubes (the script we talked above), the right part is a hand playing the sound holes. The 于 inside is free reed.
the blowpipe becomes two turning curves, but the top two short horizontal lines are not connected, which some experts think that the character is Deficit or Deficiency: the air flow is not strong enough to make sound. I will discuss the character Deficit in a different post.
a bamboo head is added over the 亏 to indicate that is made from bamboo, which is instrument, not an adjective.
similar to older scripts except small seal script. Right now the meaning of the character is not music instrument any more.
which is the wind music instrument, the original meaning of the character.
Similar to big seal script.