There is a thread over on the forums asking what mech was the first with an AC/20.
I would have chimed in with the actual answer, but I'm banned, maybe one of my readers will do some copying and pasting ;)
I have a database with every single canon unit ever, and every single detail about them, 100% canon.
So, I can run a quick SQL query (<1 minute creation time) to determine that there are:
41 BattleMechs that have 1 or more AC/20's (and 40 with UAC/20s).
Sure enough, the VTR-9A Victor, coming out in 2508, was the first mech to mount an AC/20. The VTR-9A1 also came out that year, and has one.
HOWEVER, the AC20 came out in 2500, which seems unlikely that the first mech to mount such a weapon came out 8 years after the weapon itself.
The next chassis that had an AC20, was the HBK-4G in 2572, so apparently this weapon wasn't very popular.
We do know the Lyran Commonwealth created the AC20, so the first mech to have one, is probably one of their assault mechs around that time. So let us look at mechs that came out in or before 2500 that could handle an AC/20 (lets say >55 tons).
Von Rohrs (Hebi)
Tell me, which one of those stick out to you? The Banshee huh? Well, the Mackie as well. Both of which have AC10s. The Mackie has variants (8B and 9S) with AC20s. The Banshee the 3Q but it comes out in 2915.
So, we probably have a Mackie variant with an AC20, that the Lyrans fielded in 2500. It could be a field refit I guess, but I'm betting that is the chassis that had the first one.
Well, we all assumed it was a mech <evil grin>
The first unit to have an AC20 was the Hammerhead HMR-HC fighter, which came out in <gasp> 2407, 93 years before the first AC20. I guess that'll be errata'd soon. The MUL doesn't have that unit, it was described in the fluff for the Hammerhead as the early variant that just didn't have ferro-fibrous armor. I'm guessing it should be changed to 2500. So lets ignore it and see that....
The first unit to mount the AC20 was actually the Engineering Vehicle from TRO3026 which used it as a demolition cannon (which I'm sure surprised a few attackers).