1.) See Engine Block
2.) In some cases, the thrust ring is part of the motor. It may be part of the rear closure, or a flanged permanently mounted on a single-use motor. The purpose is to prevent the motor from pushing up into the motor mount while the rocket is under thrust. In absence of an engine block or built in thrust ring, many rocketeers will make a thrust ring by wrapping masking tape around the last 1/4" to 1" section of the motor (depending on motor size). Without a thrust ring (or engine block), the friction fit technique would need to be used.