This article is brief.
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.
There is also a 'Pre-Product 4' RVT-R Reader unit which is seemingly post-final and has no apparent pre-production features; it is unknown why it is labeled as such.
Correspondence to NDEV versions
Disassembling OSGetConsoleType reveals that each of these boards from PP0 to PP2-2 corresponds to an NDEV version, with PP0 being NDEV 1.0 and PP2-2 being NDEV2.0. These boards were considered the retail equivalent to these NDEV versions, using the same Hollywood revision. It is possible that the RVT-R Reader for E3 2006 used one of these boards.