zipper without zipper foot

Close look at my trouser specially the zipper . I still have my old Singer sewing machine with just  straight stitch , and no other foots, but look at my zipper lololl I love it.

this is the back of my trouser with four darts, and the front with two darts. simple waist band, and fitting to the knee, wide at the end of the trouser. It is classic one