Rings should be a a "No-Problem".
I'm 99.99% certain it's not a drop in switch. Controlled round -vs- push feed are different. "Push feed", as the name suggests, pushes the live round ahead of the bolt, extractor, and ejector until the bolt handle turns down, at which time the extractor snaps over the rim. After firing, you rotate and withdraw the bolt- a little springloaded ejector in the breechface flips the empty out as soon as the case is clear of the ejection port.
If you look at a '77 push-feed bolt, there is usually a step all the way around the breechface. In a controlled round action, there is usually no step at the bottom, to allow the round to slide up under the extractor.
On a "controlled round feed", the round is caught and slides up under the extractor as soon as it snaps free of the feed lips. You push the bolt forward, and fire the round. After firing, you turn and withdraw the bolt, and a fixed ejector in the left raceway flips the empty case out from under the extractor as the bolt moves past it on its way to the rear. These will sometimes be a slit in or near the locking lug for the ejector to move in the bolt as it passes.
Here's the question I used to answer questions like yours with: "What are you trying to do?" Are you set on converting your 1st gen 77 to push feed? Have a 1st gen 77 that's missing a bolt? Give us some context.
I won't even go into the safety on the shroud -vs- the tang.
