Where is the boken part itself? It sounds like the base, as the backs are otherwise not interchangeable due to the armrests. Yours is an LS, which is likely non-power. I've had the covers off of a couple of seats at this point. Can't recall for sure, but I think the way the seat base is setup for the adjusting levers will make them not interchangeable. Can't be sure though. Maybe the FSM will help?

How bad did it break? That is, I had one back on my Gen I's first seats fail, but was easily able to weld it back up.

Big Truck: 00, 3.5, Endeavor, 5-Spd drive line in hand!
Little Truck: 87, 2.6T I/C, MT, LSDs, Tonneau Top
Her Truck: 03, 3.8, 20th Anniv, 65k
Daughter's: 06 Eclipse, Keeping it Mitsu!
FSMs: MitsubishiLinks.com