A fiber optic telecommunications device includes a fiber optic cassette comprising a body defining a front and an opposite rear and an enclosed interior. A fiber optic signal entry location is defined on the body for a fiber optic signal to enter the interior via a fiber optic cable. An adapter block defines a plurality of fiber optic adapters and is removably mounted to the body with a snap-fit interlock, each adapter including a front outer end, a rear inner end, and internal structures allowing mating of optical connectors mounted to the front and rear ends, respectively. A removable spacer is mounted to the body, the spacer configured to expand the size of the enclosed interior of the cassette and a removable cover is mounted to the spacer. Connectorized optical fibers extend from the fiber optic signal entry location to the rear inner ends of at least some of the fiber optic adapters for relaying the fiber optic signal to fiber optic connectors to be coupled to the front outer ends of the adapters.