Wow, that is a nice one. And you can't beat the price either! What sort of cutting blade does it have? Is it a bar or one of Jacobsens disc set ups with the four blades? If it's the four blade type, I might have new replacement blades for you for the price of postage.
Yes, Jacobsen/Textron is great about getting back to your requests for information about vintage equipment. Include the serial number that is on that plate attached to the back of the deck. I see there is also a tag attached to the lower back side of the engine. I'd include that as well. More often than not, what they send is a parts list, but occasionally they'll find a manual in their archives as well. So far as actual in your hand manuals, they turn up on ebay every now and then. There is one seller in particular who lists lots of vintage Jake manuals. You have to wait and see what he has because he evidently has a huge box of them somewhere and just pulls which ever one is on the top to list.
I also have a Jacobsen Engine Service Manual that covers the 1959 and 1960 models of their engines, but I really don't think there was much different about them over the years. And it is big. That's why your email address is needed. It won't fit here. I should have done a lower resolution when I transfered it, but if you ever want to print it out, it should be a good one. Here is an excerpt from it that covers the recommended fuel and oil, and every publication or decal on a Jacobsen tank I've seen say the same thing.
There is a fellow over on SmokStak forums who I would call the guru of vintage Jakes if you ever wanted to know all there is to know.