VESPASIAN...I have quite a few 'just' like these, and I will use them after a proper cleaning / lubrication. Having said this, I will be very cautious about using an old steel spring in any watch that does not have a design that will protect the train from a mainspring failure i.e., a "Safety Barrel" or "Safety Pinion" ( as found on USA watches )...otherwise, a person is taking quite a chance on serious damage...old steel is old steel, and metallurgy has come a long ways since the early 1900's!
The old-timers tended to suggest that a new mainspring was most likely to fail during the first few cycles...after that, things were a bit more stable. In my own experience, I have decided ( as an experiment ) to not replace springs in a number of USA PW's that were, say, 100 years old...although these old fellows have seen better days, the watch in question will still run for 24+ hours, and keep decent time...of course, if one's inclined to produce the 'best' rates, a new Swiss white-metal spring's the way to go.
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