There should be two implicit variables in an SNMPv2 Trap PDU - SNMPv2-MIB::sysUpTime.0 and SNMPv2-MIB::snmpTrapOID.0.
However, only the latter is present in the traps produced by:
C:\program files\SNMP\bin>net config workstation
Computer name <snip>
Full Computer name <snip>
User name <snip>
Workstation active on
<snip>
<snip>
Software version Windows 2002 // That's XP.
Workstation domain <snip>
Workstation Domain DNS Name <snip>
Logon domain <snip>
<snip>
The command completed successfully.
C:\program files\SNMP\bin>snmptrap.exe -V
NET-SNMP version: 5.4.2
C:\program files\SNMP\bin>snmptrap.exe -v 2c -c public monkeysex.org:1620 '' NOTIFICATION-TEST-MIB::demo-notif SNMPv2-MIB::sysLocation.0 s "wooptatadooptata"
Can't reproduce this with Net-SNMP 5.4.2.1 -- traps generated by this version include a sysUpTime.0 variable.
C:> C:\usr\bin\snmptrap -V
NET-SNMP version: 5.4.2.1