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One of the oldest Buddhist stone structures in India is the Great Stupa at Sanchi, and it has a lot of stone reliefs that have this kind of thing. Here's one example: https://en.wikipedia.org/wiki/Sanchi#mediaviewer/File:Sanchi_Carving.jpg
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Here are pictures of Buddha's footprints: https://www.google.com/search?q=Buddhapada&tbm=isch
The footprints are a.k.a. Buddhapada.
This is off-topic because it is later than "early" buddhism, but your question reminded me of the "10 bulls" illustrations (which are, if not early, at least famous, and use an emptiness to represent enlightenment).
See also this (alternatively often shown as an open, unclosed circle):