The ad said "late model ss aluminum swing arm".
Yes. As in "an aluminum swing arm from a late model SS."
SS is a model designation and an acronym for Super Sport, while SSA or SSSA are acronyms we use around here standing for Single-sided Swing Arm or Single Sided Swing Arm, respectively.
Late model is relevant because there was a wheel base change in the 1999 re-design. Aluminum is relevant because some of the newer little SS models had steel swing arms.