You could go buy a new oval 2 blade disc. Or you could buy a new 4 blade disc, both will work.
If youve got thick grass, or the grass is often wet/damp, the 4 blade helps get the catcher full. Especially with the high lift blades.
Id say it would have had the old round stepped disc on it and it would have been cracked, hence missing.
Id buy a new washer while you were there.
Wont be cheap, over $50 id say if you include the blades.