This is the draft gear slot with key detail designed for use with my bolsters with a provided slot. There are enough keys provided to do 4 cars. These should pop right into the slots on the bolsters, but be careful as these will be very delicate. Due to the nature of the material the bolsters are printed in, you may have to open up the slots a little, but they are designed so these parts should go right in. Attach them with CA or Cypox.