This is xnu-4903.221.2. See this file in:
Directory listing for xnu-4903.221.2//EXTERNAL_HEADERS
AssertMacros.h
Availability.h
AvailabilityInternal.h
AvailabilityMacros.h
Info.plist
Makefile
architecture/
corecrypto/
img4/
mach-o/
ptrauth.h
stdarg.h
stdatomic.h
stdbool.h
stddef.h
stdint.h