This is xnu-12377.1.9. See this file in:
xnu-12377.1.9 (Darwin 25)
xnu-8792.81.2 (MacOS 18, Darwin 22)
xnu-8019 (MacOS 17.0, Darwin 21)
xnu-6153.11.26 (MacOS 15.0, Darwin 19)
xnu-4903.221.2 (MacOS 14.0, Darwin 18)
xnu-4570.41.2 (MacOS 13.3, Darwin 17)
xnu-3789.70.16 (MacOS 12.6, Darwin 16)
xnu-3248.20.55 (MacOS 11.2, Darwin 15)
xnu-2782.40.9 (MacOS 10.10.5, Darwin 14)
xnu-2422.115.4 (MacOS 10.9.5, Darwin 13)
xnu-2050.24.15 (MacOS 10.8.4, Darwin 12)
xnu-1699.32.7 (MacOS 10.7.5, Darwin 11)
xnu-1504.15.3 (MacOS 10.6.8, Darwin 10)
xnu-1228.15.4 (MacOS 10.5.8, Darwin 9)
xnu-12377.1.9 does not appear to contain the file /bsd/skywalk/nexus/monitor/nx_monitor.c