This is a rather unique example of a watertight cylinder originally designed for use in the Arkmodel Type VII by a re-seller. This is not the stock Arkmodel piston cylinder and this unit is completely different. 
 
While technically designed for the Arkmodel Type VII, this cylinder is likely a bit too large for easy fitment in that model, though it may be possible.
 
Features: 
Overall length: 27"
Motor/battery compartment diameter: 80mm
Ballast tank diameter: 90mm
Single brushless motor
Dual shaft gearbox
Cylinder could be used as either a single or dual shaft unit by simply adding or removing the gearbox
Rack and pinion linear servo adapters
Three servo outputs (two rear and one forward)
Air pump ballast system
Exterior battery charging adapter
Waterproof on/off switch
Ballast control relay module
 
This unit has been bench tested for functionality but has not been water tested for leaks.
 
The ballast system uses two diaphragm pumps, one for filling and one for emptying the ballast tank. It also uses a unique waterproof membrane in place of a traditional snorkel. A traditional snorkel would be easily retrofitted, if desired. 

All functions are working. Simply add your receiver and optional AD2 pitch controller and you're set for installation in your boat.