FSCONV, the predecessor of lekseecon, connects the Level-D SDK to FSUIPC. It adds a lot of new Level-D 767 specific offsets to FSUIPC in the range 0x8B00 - 0x8BFF.
These offsets can be used by cockpit building software such as FSUIPC, FSBUS, Phidgets, SIOC, and what have you ... (however, if you are using SIOC you better use lekseecon), as well for the Level-D 767 in FS2004 as in FSX.
FSCONV is in use since March 2005 and mature; development has concluded.
User Manual (only)
Software Installer (it's free!)
User Manual included (1,9 Mb).
Is it really free of charge, even for commercial use?
Yes, it may be used free of charge, even when deploying commercial applications. However, "Free of charge" must not be confused with "completely free". FSCONV is copyrighted software, not public domain software. There are restrictions on distribution and use; see the FSCONV EULA file for details.
FSCONV is used for instance by It's Your Plane, look what they say about FSCONV: