From Собачье сердце what's the nominative of наколочке and is there a better translation than 'cap'?

from Собачье сердце what’s the nominative of наколочке and is there a better translation than ‘cap’?

nominative: наколочка; For this kind of questions, you could use this site: Ìîðôîëîãè÷åñêèé àíàëèç