A 56ohm resistor connecting the Brainstorm board to the Mac Plus logic board by way of the ferrite bead FB1. https://files.mastodon.social/media_attachments/files/111/843/677/439/172/529/original/aba1d18b48646381.jpeg
As I mentioned, the main board of the Brainstorm clips to a socket that straddles the original 68000 CPU. A 56ohm resistor connects the R1 pad of the Brainstorm board to the near lead of the ferrite bead FB1 on the Plus logic board.