Here is a WGP STO kit with a Nitro Duck / Paintball Mania Supplies E Cocker kit on it.
These kits were introduced around 1997/98 and uses a mechanical / sear with an electro pneumatic switch.
Front block rotation showing the giant solenoids.
Find Autococker parts at https://www.baccipaintball.com/parts/semi/autococker.html
Find PMS and Nitro Duck history at https://www.baccipaintball.com/oldnews/tag/nitro-duck