G5 bow sight question

gobbleinwoods

Keeper of the Magic Word
I bought a used sight which has a light. Looks like the universal G5 light. It does not come on at this time. However, when I took the top off to look at the batteries there is a little red washer/o-ring that fell out. Do I need to remove it to make the light work or is it probably just in need of batteries?
 
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