What it is and what you're working with
The E3Z-R66 is a through-beam photoelectric sensor with built-in amplifier, using a red LED light source. It terminates via a connector rather than a fixed cable, which means you need a mating connector on hand — no splicing.
How it stacks up against a cable sibling
Closest functional peer is the E3Z-T61K 2M — same E3Z series, through-beam, NPN output, IP67. Key delta: that part has a 2-metre cable, while the E3Z-R66 uses a connector. The cable version also gets a 15 m sensing distance and infrared LED; the E3Z-R66 uses red LED and its sensing distance isn't listed (suspect it's shorter or unspecified). If your panel is wired for a cabled sensor, the connector means a different mounting footprint and a wiring change at the connector. No drop-in. The E3ZM-T61 2M is a stainless steel body with IP69K and also cable-terminated — overkill for clean environments, and no connector option. Again, not a form-fit swap.
Sourcing and lifecycle reality
Lifecycle is current — factory active, no phase-out announced. That means you can spec it into a BOM without worrying about a PCN next quarter.