I believe that 1984 was the first year for the Sport version of the Stumpjumper. The specs state Special Series CrMo butted tubing but do not state a manufacturer. The 1983 Specialized frames used either Tange or Ishiwata, depending on the model. The Stumpjumper used a specially drawn, oversize , butted Tange CrMo tubeset.
I suspect the 1984 version and probably the Sport used the same tubeset but have no objective evidence to support this. If you remove the fork, there may be a stamping on the steering column to indicate whether it is Tange or Ishiwata. While it may be custom drawn, I suspect it is a version of Ishiwata MTB-D or Tange MTB.