The Pre-Production Boards (official capitalization "Pre-production boards") are a series of prototype Wii boards, their existence only being known due to SDK text strings and references in all.ini. SDK references define each of these boards as a console type; see OSGetConsoleType. Oddly, PP0 is not mentioned in SDK strings compiled into applications, and PP3 is only mentioned in the list of factory titles present in all.ini and RVL_DIAG as part of the description for the swupDiag (SWUpdate) program, which is apparently intended for PP3 boards.
PP0: Found in SDK header file.
Pre-production board 1, 2-1, 2-2: Found in SDK strings present in all Wii applications and games authorized by Nintendo.
PP3: Factory test listing
As the swupDiag application present in RVL_DIAG is apparently intended for PP3 boards, and is compatible with Dolphin, it is known that the PP3 board is similar to retail hardware.
No further information about these boards is known.