A new pair of files join a growing list of PowerBook 1400 fix files from Apple (as listed on my Apple Links page):
The PowerBook VM Tuner is a system extension that enhances system stability on PowerBook 1400 models when running memory intensive applications. While it comes with PowerBook 1400s, you may need to reinstall it manually if you do a clean install of Mac OS 7.6.
There is also an ATA Flash Extension for the 1400. This fixes an error in the 1400's ATA driver. Without it, "when you insert an initialized Flash card, the PowerBook may erroneously prompt you to initialize the card, even though the card is properly initialized and may even contain data."