Mercenaries played a little loose with mech introduction dates in that it allowed you to encounter mechs before they were supposed to be available. I wouldn't be surprised if that was done again here.
As I recall, it was mostly specific variants that actually were earlier than the "prime" variant, usually prototypes. The
Mauler can be seen early, but only because it's PGI's proxy variant for the
Daboku, for example – admittedly, we probably shouldn't be seeing a lot of those either considering it never actually went into production, but it did technically exist around the time they start showing up – and there's a specific
Wolfhound that was earlier than the "base" WLF-2.
Sometimes an anachronistic hero ’Mech shows up in shops at random, regardless of if it was actually canon to begin with, but other than that...