Boulder with iron loop?

The other day EG unearthed many building stones from among the roots of a giant conifer in our garden. We have been here more than 30 years, but still find caches of great rocks and stones and strangely shaped bricks. The orange-coloured one on the right has a cylindrical shape on one corner.

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We are rather curious about this boulder with the iron loop in the top. Any ideas?

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