I printed the relevant pages from the manual re: belt tension and so on and took it out to the mower.
There's no tension rod adjuster. Somebody's modified this thing, it would seem, so I'm done.
The blade engagement lever pulls a cable with a spring that pulls a small idler closer. That tightens the belt so that the blades can spin, except when the lever's the other way the belt is so loose it falls off, assuming the larger idler that looks a little cockeyed isn't a contributing factor. I see no cable length adjuster to permit the belt to be tighter when at rest.
I also saw what looked like a spindle brake on the wrong side of its spindle, almost acting like a belt keeper. Someone would have had to remove either it or the spindle to locate it on the opposite spindle side. So again, I'm all done here.