One of the many indications that Assad is responsible for the chlorine attack in Douma at April 7, 2018 are the wheels which are attached to the harness which enclosed the yellow steel cylinder. Photos taken at both apartments show what very likely are wheels. The wheels arre used make pushing the cylinder from helicopters much easily.
The wheels in the red square are attached to the harness of the cylinder found in a yet unknown location in Douma.
Here is another photo.
The photo below show similair wheels found on the balcony of a second apartment which was hit by a cylinder dropped by a helicopter.
The photo below shows another yellow cylinder with wheels. The photo was made in Bustan al-Qasr at December 10th 2016. Other photos do not show the axis and wheels. So likely someone placed this for the photo(source)