It is my understanding all the PAS have flex shafts but cant confirm that. Never had one apart. "S" series have solid steel shafts along with the DSRM cordless trimmer. Echo has a lifetime warranty on the flex shaft so no sense spending the extra money on a solid shaft.
EXCEPTIONS:
• For two-stroke engine powered products, the electronic ignition module, flexible drive cables, SRM solid drive shafts, and TC tines
are warranted for the life* of the product on parts only.