Karen, Went to TS this morning and they had their lawn tractors put into a conex box for the winter. But I did some digging online and you are not missing any bolts from what I see. The part number for your seat is Cub Cadet 757-05009B . When I run that number though Ebay I find some used ones with pics of the underside. Any of them that have bolts only have the one. There is one listing that has the seat only and not the base and I can see clearly that those inserts are installed into the seat in a manner that they wont come out, or do not need the bolt to secure them.
I studied a little more and realized that the base assembles to the seat by sliding it in from the front of the seat. -If you look at your pic you can see how the "T" openings at the front of the base allows the inserts through.. I think that bolt is installed to keep the steel seat base from sliding too far forward, or coming off completely. If you were to have to replace the seat, that bolt must be removed in order to slide the old seat off and the new seat on.. See the listings below
Listing with seat only showing the inserts clearly:
https://www.ebay.com/itm/2268674871...U0PtN1Y0Vrh5Tzl8tK1gVJx29zwyulRoaAmMCEALw_wcB
Listing with complete seat and base with one bolt;
https://www.ebay.com/itm/3870921809...gzLrkiDzek7zaPmVEOEuxPB87i0399S4aAqhYEALw_wcB
Also note that the Partstree breakdown only shows one bolt, a 3/8-16 by 7/8" long.
I studied a little more and realized that the base assembles to the seat by sliding it in from the front of the seat. -If you look at your pic you can see how the "T" openings at the front of the base allows the inserts through.. I think that bolt is installed to keep the steel seat base from sliding too far forward, or coming off completely. If you were to have to replace the seat, that bolt must be removed in order to slide the old seat off and the new seat on.. See the listings below
Listing with seat only showing the inserts clearly:
https://www.ebay.com/itm/2268674871...U0PtN1Y0Vrh5Tzl8tK1gVJx29zwyulRoaAmMCEALw_wcB
Listing with complete seat and base with one bolt;
https://www.ebay.com/itm/3870921809...gzLrkiDzek7zaPmVEOEuxPB87i0399S4aAqhYEALw_wcB
Also note that the Partstree breakdown only shows one bolt, a 3/8-16 by 7/8" long.

