While the SA-35 comes with 15-round magazine (Mec-Gar) these have been available for quite awhile, and 13 rd mags are perfectly useable. This S.A. Camp article describes the differences between the two.
Mec-Gar 9mm Hi Power Magazines: 13 vs. 15-Round
On the left is a Mec-Gar 13-round Hi Power magazine and on the right, a 15-shot one. They are the same length.
Mec-Gar 9mm Hi Power Magazines: 13 vs. 15-Round
On the left is a Mec-Gar 13-round Hi Power magazine and on the right, a 15-shot one. They are the same length.