The board is a small module which plugs into the 8271 socket of a BBC Model B. It allows the use of both 1770 and 8271 floppy disc controllers, if present on the board.

I think the green lead probably went to a switch originally to select between 8271 and 1770 mode.

I haven't got it working yet. I think the board may be damaged as there is evidence of something having been removed in the bottom right-hand corner and one of the capacitors is broken.