Hi mow master! Welcome to the group. Wow, I get pretty excited every time I see that somebody else has a vintage Jacobsen.
I looked up your model number on that site Austen listed. Scroll down the list on it to "4-Blade Rotary 18" and you'll find your model number, 31807 in a second list. Then enter serial number 9416 and you'll find that it was manufactured in 1964.
Tell us more about your old Jake, and maybe post a picture of it. Does it have a steel or cast deck?
Parts lists and owner's manuals show up every now and again on ebay. I've also had great luck in contacting Jacobsen/Textron. They have some manuals for the vintage mowers in their archives, and have shared some for machines I have. You just need to provide them with the model and serial number with a description of what you're looking for,