Leica 50 Summicron Dual Range rattle
...are used to allow a certain amount of travel / play with elements not meant to be tightly attached. I'm not sure, if this washer is a member of those features allowing the lens to be set to close range focusing.
You mentioned the "jiggle". Unfortunately I no longer have my dual range 50 but I can't recall any overly loose barrel components or any play I'd describe as a "jiggle". If you know a knowledgeable Leica dealer nearby, you might want to show and ask there. Of course you could always mail the lens to Leica and ask for a check or for servicing. Naturally that will cost - but you'd be sure to have a freshly serviced lens.
You might want to google for any Leica service manuals for oder lenses. Not sure, if these are readily available, but they'd probably show exploded views of also that dual range 50mm.