Hi Ray, thanks for the comment on my guestbook.
I recently stumbled upon this link and it shows a SMC Takumar 55 1.8 (exact same as mine) displaying some sort of radioactivity.
Radioactive lens (SMC Takumar 55/1.8) measured with Polaron-Pripyat and RKSB-104 dosimeters - YouTube
Not that I care about the radioactivity all too much. It's still a good lens.