The model number that you posted does go to a mower with a 42 inch deck, but that model number doesn't go on that mower. Seems like the model number tag is on the bottom of the seat mounting plate, that plate has come off of another mower.
Would you mind posting the model type and code information from that tag next to the flywheel. Sometimes I can cross that back to a mower or series of mowers that could be a match.
Is this what you found from the model number I gave you that was on the motor?models 134Y450F129 and 134Y470F336 are possible matches for your mower.
That is what I found by doing some digging on the MTD Dealer site. On some of their models they list what engine was used on what model. The code part of the model number gave me a basic starting point for year.
The hook that found is what the springs connects to. the short slightly off straight part is where the spring connects the longer more 180 degree bent end hooks over the top of the axle housing in the grooved area on the right side of the axle.
One the first notch down the deck should move rearward to tension the belt, and then not move backwards since the deck belt should be tight and then the deck should just move downward as the deck is lowered. There may be an adjustment rod connected to the lowering lever that will change the point that the belt tightens.
At the off position, are both of the pulleys pushing against the brake pads hard enough to stop the blades rotating ?
Good bet they will be hanging 1/2" to 1" below the pads
Every second one of these I work on, some one has put a spacer block under the brake arms to make up for the fact that the deck is lower than it should be, otherwise the blades never really stop spinning and the belt tends to burn on the drive pulley.