An actual Singer 15 (unlikely) may not be what you're looking for.
If it is a 15-90 or 15-91, they are very good machines well worth getting. The serial # will be on top of the base in the front right corner.
If by some long shot it is a Singer 15NL or 15CH, take a pass on it. These were later made "nostalgia" machines made in Taiwan and China and have a poor reputation. The model and serial #s for these will be on the under side of the cast base.
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