I can't quite see which plate you are referring to but i can tell you that the seat springs are for the Rest-O-Ride seat. It was an added option offered on the hundred series as compared to the standard single spring flip up seat. There are a few places that still sell the replacement parts for the seat if you need any.
The large rectangular plate under the spark plugs is to access the valve push rods. The cam is down below that and the rods go through a hollow area in the block behind that cover and then go up through the head to the rocker arms that are under the valve cover on top of the engine. If you need to do any head work or replace the rods because they are worn or bent, it would be near impossible to slide one in from the top and find the correct hole in the bottom for it to go in without having that removable cover.