Suspension Failure

The fact that they came loose probably has more to do with the previous owner not doing things up tight enough than a lock washer failure. Nylocks are (mostly) more reliable, at the expense of mudding up exactly how tight you're torquing the fastener, because of the deformation drag. It also changes if you reuse them.
There should be Two studs, however quite often the stud and nut come out together as if they are a set screw. Hence when you bought the elbow it was missing one. Standard bolt are Grade five high tensile. If you look at the top of the head they should have three raised lines pointing in a star shape from edge to centre. No lines means they're not high tensile. More lines (usually six) means they are probably grade Seven which is tougher.

With regard to the spring washers, they can be put on upside down. They are designed to slide on tightening and dig in on undoing. Wrong way around they gouge on tightening and come loose! You can also get them in varying widths for better grip. In the de dion size the wider ones are also easier to see!
One more point... There aren't any nuts because there isn't any room to put them on. The holes are too close to the tube. Personally I'd shorten the screws so they end at the hole edge or slightly inside as crud on the thread will cause them to seize on removal and being small they may snap.