Jim/Tom,
Thanks for the additional information. It appears to me that the racetrack was extended but from that drawing it also looks as though "Pandy" was a bit bigger than what it morphed into.
Courtesy of Melvyn Morrow, here's the scoop of the proposed 9-hole addition to Musselburgh in 1895 that was never built. Since this was five years after the decision of the Honourable Company of Edinburgh Golfers to move to Muirfield, it wasn't seemingly related to that move.
What instead makes more sense is that the 1896 Open was awarded to Muirfield, which was quite a jolt to the folks in Musselburgh, including the 2 or 3 remaining clubs who played there. I would think this effort was trying to make the old Musselburgh course relevant for continued tournament play as the world was moving to a standard of 18 holes.