Welcome and congrats to the choice of bike! Just fire away with questions, that is the best way both for the student and teacher to learn more! Please make relevant headlines to ease for others to find topics later. Best is with a lot of different questions, separate dem in topics.  

A special round head carriage bolt is fixed in the frame bracket with a small protrusion or lip. The saddle tongue is sliding plainly on the bolt, and a zirk grease fitting on the tongue is for lubricate that. 

http://walkermachine.com/ will set you right, I didn't find a pict. of that particular bolt there, but he has many nice pictures for identification of details. He might have heavier springs as well. Both outer and inner can be found stiffer and could be matched to suit the bouncing rate. if not, he ought to know where to find them. I read somewhere that there was spring rate options from the factory.

The saddle plate on the frame has 3 holes where the spring can be placed. Stiffest adjustment is to have the yoke straight up and down, the more it leans the softer it becomes. That can be handy if you find your new springs too stiff.

It may also be possible to move the saddle a bit on the front tongue, to get the yoke even more upright. And when the nuts under the tongue is loosened a bit, the seat leather can be stretched with the adjusting screw underneath.

http://www.oldtimerservice.net.pl/ might have springs.

You'll find nice pictures for detail identification and of course purchase on, for example http://reno.indianmoto.cz/index.php, and http://www.zorros.net.au and many other sites.