function of metal ring in a copal shutter

[studio347]

Hi,
What's the function of thin metal ring which is placed between a copal shutter and the front glass?
I find one yellow color ring at a schneider lens and 3 black rings(together combined) at a rodenstock lens.
Can I use the lens without the ring(s)?
Thanks in advance!

[Ellis Vener]

they are shims installed by the lens maker to create the correct spacing between  the front and rear sets of elements for the individual shutter the sets are installed in. For best optical performance leave them installed.