The first detailed dive into Solaris was the box set, which was set in 3055. Before that the most detailed look at the world would have been the novel Warrior: En Garde, but it was focused on Justin Xiang's time there and all the pertinent information was repeated in the Solaris box. I don't recall converted exoskeletons being mentioned in the novel but it's been a long time since I read it.
In terms of pre-Clan Invasion BattleArmour technology in the Inner Sphere, the Star League had the Nighthawk Power Armour (Light) suit which offers the protection of BattleArmour while only carrying anti-personnel weapons, but it was never produced in great enough numbers to be deployed outside of special forces, and while ComStar ended up with a stock of them, they were never able to prouduce Nighthawks. In the 2900s they did introduce a homebrew equivalent called the Tornado, which was severely limited in deployment until the 3060s. Nighthawks were practically unknown in the Inner Sphere until the Helm Core was disseminated. The Bounty Hunter wears a customised Nighthawk that was presumably discovered in a lostech cache at some point.
Meanwhile, exoskeletons are common enough tech to have been listed in TRO:3026.