The British ring circuit and fused plug were introduced in the late 1940s, but prior to that we had a multitude of standard connectors.
Plugs and sockets came in three current ratings - 2, 5, and 15A - without fuses. All were available in 3-pin grounding versions, each having round pins in a triangle formation, the only difference being that the the higher-rated connectors had larger diameter pins spaced farther apart. The earth (ground) pin was slightly larger and longer than the other two.
The 2 and 5A versions were available in 2-pin non-grounding versions as well. (There were also 2-pin 15A types installed back in the 1920s/1930s, but these went out of production.)
Unlike American plugs, a 2-pin plug will not fit a 3-pin receptacle as the line/neutral pin spacing on the grounding & non-grounding versions is slightly different. This applies to both 2A and 5A types.
The intent was that 2A plugs would be used mostly for portable lighting, 5A for general small appliances up to about 1000 to 1200W, and 15A for heavy appliances, such as a 2 or 3kW room heater or an electric kettle.
The IEE Wiring Regs. specified that a branch ciruit not exceeding 15A could feed any combination of outlets based on the following assumed current demand:
* 15A socket: 15A
* 5A socket: 5A
* 2A socket: 0.5A
Thus a 15A circuit could feed one 15A recept. or three 5A recepts. It wasn't at all unusual to find more than three 5A outlets on a circuit, however.
It was rare for there to be more than one 15A outlet in each main room, not always including the bedrooms.
Fixed lighting points could (and still can) be wired on any branch circuit up to 15A, although 5A circuits dedicated to lighting were more usual in residential work. Each light was calculated as the actual connected load or 100W, whichever is greater. (This same calculation still applies today for lighting.)
The multitude of plugs meant that various combinations of adapters were widely used. One common type plugged into a 3-pin 5A socket and provided a 3-pin 5A socket on the front and two 2-pin 5A sockets on its sides. Another common type adapted a 15A outlet for 5A plugs.
These connectors didn't disappear quickly after the introduction of the ring and the fused plug. They were still in widespread use in the 1960s, and I saw a lot of them in older homes when I was growing up in the 1970s.
Some 5 and 15A 3-pin plugs made in the 1950s onward were designed to carry a fuse to provide additional protection (different type to that used in the 13A ring plug).
The 2A version survived into later years for wall-switch controlled lighting outlets, although such installations are now very rare.
The 2-pin 5A plug has survived as the standard for electric shavers, the special xfmr-isolated outlet being the only recept. allowed in a bathroom.
The 3-pin versions of all three types are still manufactured. They have remained in common use in some former British colonies, such as some African countries. They are also still popular for theatre lighting here, where a blown fuse in a 13A plug high up in the rigging could be most inconvenient.
I still use the 5A types regularly in my electronics workshop. They are ideal for powering a lot of low-power apparatus, being much less cumbersome and bulky than the 13A plug.