In 3064, Duke James Sandoval realized too late that the strike against House Kurita could backfire. In addition, the FedCom civil war gobbled up Commonwealth military output, forcing the desperate Duke to search for alternative sources. Seeing successes with refitting old designs, Duke Sandoval directed Robinson Standard Battleworks (RSB) to assess older Battlemech designs for quick refitting. A frenzied study suggested that the Ostsol was a likely candidate, a hand-me-down 'Mech received in quantity by the Draconis March Militia.
Like the FWL refit, the 6D-Ostsol replaces the Vlar 300 power plant with an XL model. This frees up significant space with which to upgrade weapons without compromising speed. Indeed, the use of triple-strength myomers allows the Ostsol an extra burst of speed - literally from the heat of battle.
The Draconis Militia and the Robinson Ranger regiments received most of the refitted Ostsols, with the rest going to units damaged in the Draconis Combine invasion.

Source: Technical Readout Project Phoenix
Armament:
2 ER Large Lasers
2 ER Medium Lasers
2 Medium Pulse Lasers
2 Small Pulse Lasers