Parts for nelson based pumps built by Earon Carter of Carter Machine. These include parts for the Buzzard, the comp, the tri-car / tricar and more.
And articles on Carter Machine at:
http://www.baccipaintball.com/oldnews/tag/carter-machine
Many Carter Parts are totally propriety and some are not compatible between generations.
Old pre 90 Buzzards, Comps, Ducks and Box Guns use standard 9/16 inch by 18 threads per inch valve retaining screw and the powertube is hole is standard ID.
Carter pumps of the 90s use 9/16 inch by 18 tpi valve retaining screws but the interior dimension is larger. Not sure when this change occurred?
Newer Carter pumps (maybe 2005 onwards?) use a larger outer diameter valve retaining screw.
The valve retaining size on these powertubes are 5/8 inches x 18 threads per inch (tpi). Some are 20 tpi though
And checkout this thread on mcb for more information on Carter Power Tube Valve Retaining Screws