Found this weird ceramic block plugged into my dining room outlet, any ideas?

That is a **Fedtro Hexo-let**, which was a pretty common multi-outlet adapter back in the 1960s. The heavy, textured feel you're describing comes from the fact that these were often made with a die-cast metal shell over a phenolic plastic core. They were built like tanks compared to the cheap plastic power strips you find at big-box stores today. The angled side and the specific layout were designed to allow you to stack up to six two-prong plugs into a single duplex outlet. The round hole in the center is for a long mounting screw. You would remove the original wall plate screw and use a longer one provided with the adapter to secure the whole unit to the outlet. That way, when you pulled a cord out, the entire block didn't come flying off the wall. Those faded numbers and letters on the slots ussually indicate the amperage and voltage ratings, which were typically 15 amps at 125 volts.

Since this was in an old rental, it makes total sense. People in the mid-century era were suddenly buying tons of new appliances and gadgets but lived in houses with only one or two outlets per room. These adapters were the go-to solution before modern surge protectors became the norm. They even came in diferent colors, though most of the ones left today have been painted over several times by landlords, giving them that ceramic or stone-like texture.

If you're planning on keeping it, just be aware that these aren't grounded. They are strictly for two-prong, non-polarized plugs. I wouldn't run a space heater or anything with a high draw thru it, but for a few vintage lamps, it's a cool period-correct piece of hardware. Just check the internal copper contacts for any signs of arcing or corrosion before you actually put it back into service.

That explains why there was a hole right in the middle! I was wondering if I was missing a piece. I noticed the texture is super thick, prolly from decades of "landlord special" paint jobs.

If I wanted to use this for some LED string lights or a clock, do I need to worry about it being unpolarized? I noticed one slot isn't wider than the other like the outlets on my newer power strips. I’d hate to fry somethig or start a fire just because I wanted to keep the vintage vibe.

You'll want to check if your LED plugs are polarized. If one blade is wider, it won't fit without modification, which you shoud never do. Most modern electronics need that polarized path for safety.

Watch out for sagging. Even with the center screw, the weight of that metal shell can strain the outlet contacts behind the wall. I’ve seen these old taps get hot enough to char the drywall beacuse the internal connection got loose over decades. If you hear any sizzling or the plugs feel 'soft' when you push them in, just keep it as a display piece. Those old copper leaf springs don't last forever.

Man, those Fedtro units are basicly artifacts now. If yours has been painted over a bunch, just be careful that no paint actually got inside the slots. I've seen them where the paint bridges the gap or coats the internal clips, which is a fire hazard if you try to draw a load through it. Also, give it a good shake. If it rattles, the phenolic core inside might be cracked and the metal parts are loose. If it feels solid, you're probably fine for a clock, but keep an eye on it for any weird smells the first few hours it's plugged in.