First contact with Covenant was Feb 2525 with Pearl Harbor style glassing of Harvest. SPARTAN IIs were briefed of Harvest that Nov upon return of their first real operation (snatched rebel general) ...they were only 14 yrs old and had been in the program since age 6. Later that same month they were introduced to Project MJOLNIR. Thats 27 yrs (and 207 ground engagements w/ Covenant for SPARTAN-117 per his CSV) before Battle of Reach and subsequent discovery of first Halo, that's a wide window of time and conflicts for Halo Wars to work within. SPARTAN IIs were issued first MNOLNIR with shields mere days before Battle of Reach / Halo, therefore any Halo Wars MNOLNIR armor should be unshielded.
All this came from the first book but there is no use of armor Mark #s so I can't confirm or deny the Mark designations given on wiki and elsewhere.