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I don't have that particular strobe but could it be separators for the AA batteries?
Might they be the plastic holders on the back of the cap? See my photo of the back of the cap of my YS-03; look at the upper right & at the lower left of the photo.
The function of these plastic pieces (my speculation) is to assure that no electrical contact is made with the AA-size cells if you put the cells in the wrong way. The plastic piece makes sure that only the button on the "+" can contact. As long as you're careful about always putting in the batteries the right orientation, no issues with not having the plastic pieces.
Glad to hear your YS-03 is still happy!