Basically everyone Igor encountered with had his face cast mask; His colleagues, his friends, the prostitutes etc; He gave this mask to people and they could keep it if they wanted to; Obviously Dafne kept it and used it for the face riding;

Igor loved to "claim" people as "his" by putting the mask on them; it was also a way to kinda dehumanize them, because he couldn't care less about most of them so by hiding their faces, he liked to emphasize his superior position in their hierarchy;

Igor also found it kinda fun to put the mask of his tired old face on a beautiful young body; It was his way of self-irony; obsessed with youth and young bodies he loved to make young people get naked and wear his face; he found this view bittersweet because he never got to keep his body intact because when he was in his 20-s he had to go to war to pay off his debts and stuff; weird way to have fun but whatever;