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Paul, would this domestic dc be 120V to ground or 240V to ground?
At the times I'm talking about, the British 3-wire DC systems were 240 to ground, and thus 480V between outers. (Or thereabouts: As this was before standardization, the specified nominal voltages were between 200/400 and 250/500V depending upon the district.)

Normal residential services were 2-wire 240V, with half the houses tapped from the positive pole and half from the negative. Commercial premises could get a 3-wire 240/480 service.

I understand that there were some 120/240 3-w systems in the very early days (e.g. late 19th/very early 20th century).

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Where did the distribution systems get the DC? Local power plants with DC generators?
Yes, a BJ has mentioned, DC is not easily transposed to higher voltages and the low-ish voltages severely restricts transmission range. Thus DC systems were only practical in urban areas with loads close to the generating station.

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When did other countries in Europe switch over to 220V/380V 50Hz AC? And were there other systems in use in between? (I know the 127/220V system was, but when was that phased out?
I know that parts of France had 127/220V in the past, and this is a question I posed on a French forum some time ago.

A couple of people told me that there was a gradual change from 127/220 to 220/380 after World War II and into the 1950s. But apparenly some areas still had their 127/220 services in the 1960s and even 1970s -- These with actual 127V outlets, not the neutral-less two-phases to give 220V as still exists in some parts of the Continent.

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Of course you could get AC/DC radios well into the late 60s & early 1970s (transformer-less 5-tube "all American 5" sets). Reason was they were cheaper because manufacturer could avoid the cost of a transformer.
In Britain at least, the transformer-less design survived longest in television sets. Live chassis sets with half-wave rectification were still common in the 1970s, which isn't surprising considering that TVs were also one of the last mainstays of vacuum tubes.

By the early 1980s, TV design started moving toward switched-mode (chopper) power supplies. Many of these designs resulted in a chaasis which floated in potential to about half the mains supply level, no matter which way the supply was connected.

So with the earlier sets, one could at least guarantee a zero-volts chassis by observing correct polarization, but on the new sets the chassis would sit at 120V no matter what.

Isolation transformers at the ready! [Linked Image]