@RWB,
This interposer is not the same or (not even close to) as P1 Red Board.
This interposer board has access to ALL pins of P1 module.
However it doen NOT have ANY other circuit other than a one to one mapping to the 1mm-pitch-0.5mm-dia-holes-for-pin-row-headers.
For example: I would use the interposer board to develope my own version of P1-Red_board for prototype purposes only. For production, my eagle footprint provides for direct soldering of P1 Module on to the P-Red_board-Type-Of-DIY_board.
See footprint below:
