Unexpectedly, I found a reference to a single-wire compensator in Hans G. Wägli's "Hebel, Riegel und Signale": In 1885, at least, such a apparatus was present at a small station in Switzerland, shown on the following plan from page 176 (click opens a somewhat larger image):
"Abschluss-Signal" is the term for a home signal, which was at that time still an "outer home", i.e. far enough from the first set of points so that a train could stop before the points if the signal was not cleared.
It is interesting that that Swiss compensator was located about a third from the signal. I do not know why one would choose this ratio—in principle, one could also make the two coupled rolls of equal size, or of any other ratio.
The plan also shows a compensator on the opposite side. That compensator is not located so nicely at one third—however, the plan is not really to scale.
I do not know where these compensators originated—are there any in Britain, or have there been? I'll scan Wägli's extensive citation list, maybe some interesting article or book turns up that might explain this.
Last, but not least, there are also "Annet-Schlösser" in this plan. These are obviously Annett's locks, which lost one T somewhere between England and Switzerland.
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