The Servo pen appears to have a reverse lever. However, it is actually a large U shaped bar that is not attached to the barrel at all. The rigid metal U shaped handle runs through a split in the far end of a rigid pressure bar. The bar sits underneath the sac rather than on top. Lifting the “lever” or exposed part of the U bar results in lifting the pressure bar from the back to compress the sac by a pulling type motion.
lifting the “lever” brings the flat pressure bar up from the back. 
Added line to illustrate the U bar position 
The entire mechanism is loose. Here it is slid towards the section, showing the pressure bar lifted up, also demonstrating how the U bar runs through it.
