Do the exhaust valve guides have snap rings to limit downward install or later creep?

The copper things sound like bronze valve guide inserts. You drill out the guide, and ream them out to the insert size, then insert and knurl the insert to the guide, then ream the insert to the correct stem diameter. There is also one with bronze wire. Bronze is sort of self lubricating, a superior guide liner.

I'd drive out the old guides and have new exhaust at least installed to get the snap rings. THese engines are known for dropping the exhaust guide. Buy the best sten seals you can find. I have no real rec on these, and would like to hear one...