Those lines were used by modelers who build in the style of ship modellers - what they call "bread and butter", laminations of thinner timber cut to those lines and glued together. There used to be an Italian magazine called "Style Auto", I believe that this drawing, and others like Alfa 33 Stradale, Alfa Montreal, Alfasud (bit of a theme going, they had) were published in it.
Thanks a lot everybody for kind words. I didn't know, that such prints were available in Auto Style magazine, This print comes from Polish book "Nadwozia samochodowe" (Car bodies) by Janusz Pawlowski, it was issued in 1978. But I keep digging libraries, second-hand bookshops, auctions and if I catch something, you will know first