I loaded the level image in the avatar... so it's easier to explain... the point is that I already solved this level before in another Sokoban game (it was running from a 5.25" floppy drive of a PCjr ... it was early '80... processors were 8088 .. standard speed 4.7 Mhz)... :D what a nostalgia...
<_< I'm not playing BOX but Sokoban Pro version 1.1.
any other version of the game have all one less wall box that allows the boxes to be feeded from the back... this in sokoban pro 1.1 is the only occurrence I know and you too were stuck at level 73.
I upgraded at 1.2
this level diappeared...