> Firebird 1.5 GBAK expects to find fbclient.dll in the \bin directory
> beneath the firebird_1_5 root.

In other words, GBAK calls fbclient.dll using "load-time dynamic linking",
i.e. fbclient is located using usual schema in these places
1. The directory that contains the module for the current process.
2. The current directory.
3. The Windows system directory.
4. The Windows directory.
5. The directories listed in the PATH environment variable.


In yet another words, GBAK does not use registry entry
HKEY_LOCAL_MACHINE\SOFTWARE\Firebird Project\Firebird Server\Instances
to locate client library.

Ivan