I have a P19 and am familiar with the chainplate mounts. The chainplates are each secured to the sides of the hulls by three pan head machine screws which pierce the hull skins. These screws go through the chainplates and thread into a gudgeon plate which is bonded to the inner hull surface. Removing the three machine screws should release the chainplates which will slide upward through the small exit slots. You can prevent corrosion of the chainplates in the future by ensuring that the top of the chainplate where it exits the hull and the screws are installed with a flexible sealant. If saltwater gets into the pocket that contains the gudgeon plate, it will aggressively corrode the stainless steel parts. Remember that so-called stainless steel needs exposure to air to remain corrosion-free. If immersed in an electrolyte like saltwater and depreved of air it will definitely corrode!

Jimbo