hello99: When I started learning these halachos I was told ein machzikinan b’isura. The sefarim do not bring any such chazaka even though such tefillin were very common for a very long time. It is lishma unless proven otherwise even if they were — my teacher had this shaila, though they were pasul for other reasons — bought at a flea market from someone (idk whether Jewish or not) who found them in his basement. If we needed proof positive that the sofer was writing lishma at that moment, no one’s tefillin would be kosher.
