The 40m-IFO vacuum envelope doors are sealed with dual viton O-rings and they are pumped through the annulos lines.
This allows easy access into the chambers. The compression of the o-rings are controlled by the o-ring grooves.
The OOC (output optic chamber)'s west side door has no such groove and it is sealed by just one single O-ring.
We have to protect this O-ring from total compression by 3 shims as shown below.
There were control shims in place before and they disappeared.
Let's remember that these shims are essential to keep our vacuum system in good condition.