The citation for it on the website of one of the creators is:
Rai, D. C., Firmansjah J. and Goel, S. C. (1996). “SNAP-2DX: Structural Nonlinear Analysis Program for Static and Dynamic Analysis of 2D Structures; (MS DOS Version).” Rept. No. UMCEE 96-21, Univ. of Michigan, Ann Arbor, MI, Aug., 71 p., Sponsored by a group of design firms on the West Coast.
Doesn't sound like anything you can't get from a number of different programs, but maybe the OP knows something I don't...