The public symbol file NTKRPAMP.PDB for the original release of Windows 10 tells that the kernel is built with the CLFSLSN.H header at


and draws from it the following type definitions:

Line Number Type
68 union _CLS_LSN
74 unnamed struct offset in _CLS_LSN

The header CLFSLSN.H is not known in any Device Driver Kit (DDK) or Windows Driver Kit (WDK).