question about my tig machine..Econotig

I purchased a used Miller Econotig from an auction. when loading it
up in the back of my truck, I disconnected all of the cables. Upon
unloading it, I noticed an O-ring laying on the floor of my truck.
The ring is about the size of a half dollar. I can not find any place
on the machine that an o-ring of this size might go. I have reviewed
the manual and it does not show a ring this size. It does show the
small ( dime size) ring where the tig cable would plug in...but
nothing this size....I think the ring I found my be from something
else I hauled in my truck some time ago. anyone know if the Miller
Econotig uses a ring this size???
Thanks
Bruce