The SI for MBH98 listed 34 tree ring series that were not actually used. This was acknowledged in their 2004 Corrigendum which provided the following implausible excuse:
These series, all of which come from the International Tree Ring Data Bank (ITRDB), met all the tests used for screening of the ITRDB data used in ref. 1 (see ref. 5), except one—namely, that in 1997, either it could not be ascertained by the authors how these series had been standardized by the original contributors, or it was known that the series had been aggressively standardized, removing multidecadal to century-scale fluctuations.
A number of the unused series were generated by Schweingruber. Other Schweingruber series arising from the same collection and publication were used in MBH98, making the application of the Corrigendum excuse to the Schweingruber series highly implausible to say the least. Other unused series were generated by prominent dendrochronologists in published articles, whose methodologies were either published or easily available to MBH. Whatever the actual reason for the omission of the 34 series, the above reason seemed highly implausible.
On a small whim, I cross-checked the Mann et al 2008 against the Corrigendum exclusions.
25 of the 34 series said to be unsuitable in the Corrigendum were used directly in Mann et al 2008; 4 excluded Schweingruber MXD series are used in the MXD grid. The only series not used in Mann et al 2008 are 4 Schweingruber RW series and one oddball (chil016, a Roig series – other Roig series are used.)
For 23 of the 25 series, the versions used in Mann et al 2008 are identical to the ITRDB version, previously said to be unsuitable in the Corrigendum. For the other 2 (arge065, cana106), the version differences were not material.