How is the shim (coin guide) attached to the coin optic boards on igt +. Tia
Welcome to the site.

If you're talking about the plastic encoder, I don't want to go and take mine apart, but I believe it's just held in place by a couple of screws that are exposed once you unplug the wire harness that goes to the coin-in optics assembly. See the red circles in the attached photo to this post for the location of the screws.
Also I've attached a helpful photo from an old post (thanks to Stayouttadabunker for the picture) of the two different common encoders -- one for large coins like dollar tokens, and the other for smaller coins like quarters and nickels. You can see in the large coin encoder that a small coin like the nickel shown can pass right through without tripping any of the coin-in optics.