It depends. Wall outlets had the same holes Schuko receptacles have (nice for inserting contour plugs, but allows plugging in Schuko plugs), the ones used on extension cords have smaller holes, thus accepting only ungrounded and Euro plugs.
The pins of our old plugs were solid the first 5mm where they emerged the case, then they had a small slot. It looks as though they'd been partially cut in halves from the tip with a very thin saw blade. Cheap types didn't have that feature, they tend to slip out of most sockets (remember, all of them are older than 20 years now, they're pretty likely to be worn out at least a little bit).
Oldest sockets I've seen are from the beginning of the 20th century or maybe even earlier, were made of brass and white porcelaine, lokking pretty much like the one posted on the "old electrical parts" thread some time ago. These were usually wired with surface-mount 0.75 sq. mm cord, 2 single sheathed wires twisted around each other, put on porcelaine or glass insulators. If I'm ever going to see such an installation I'll take pictures, maybe I even have some (I think I took a pic of a light switch from 1924 with some cord)
