I believe I found what you are asking:
Serial: 120795A 001389
MFG ID: HD3L4E9A
Because of the age of the unit, I've had absolutely no luck in locating an owners manual for anywhere. However, I'll post an illustration and step you through the process.
The very first step is to get it on a level surface such as your driveway. The next step, and this is critically important, is to check the air in your tires. A low tire will throw you off every time. With the key switch off and the spark plug grounded raise the cutting deck to its highest position.
Using a ruler or table measure underneath the deck. Start with the blades toward the outer position and measure the distance from the blade to the ground on each side. You should try to stay within 1/8 of an inch. If the distance is greater than 1/8 of an inch. Then you'll need to make the adjustments to the hangers I will point out one illustration below.
Next, move the blades to the forward position and measure the distance and the blade tips on the front and the back. You should be slightly lower in the front, somewhere between a eighth of an inch to a half an inch. If the deck is level, from front to back it will not cut properly. It needs to be slightly lower in the front.
Please look at the illustration below, and it will point out to you the location for the adjustments are made. I am also going to post a link to a video that while not for your specific tractor, the generalities are exactly the same.
I hope this helps. Please let me know if you have questions.