Setting kMasterKey.nDeriveIterations to at least 25,000 if it's lower—were introduced in Bitcoin version 0.4.0 to enforce a minimum security level in the wallet encryption process. This means any wallet.dat file created or rewritten with v0.4.0 or later will always have at least 25,000 key derivation iterations recorded inside. If the count is less, it's very likely either from an earlier version, is corrupted, or is not a legitimate file.