It is called the L/R Piston Tool and it makes the removal and installation of the piston return spring in a 604 a simple and quick job to do.

 It is available from Transtar as part # T9003, or you can get it from the maker himself Scott Hetrick from Transplant in Atlanta. He can be reached by calling (770)263-0221 or you can e-mail him at paddlerscott@aol.com or Paddlerscott@mindspring.com.