1933 British Penny
By date, the 1933 penny (Freeman 209) is one of the rarest British bronze pennies. There was no need for circulating pennies in 1933, so only a handful were struck for institutional collections, as well as three for placement in foundation stones of buildings. It is believed that seven or so were struck in total (Royal Mint Museum, https://www.royalmintmuseum.org.uk/collection/coins/1933-penny/).
Of the three placed in foundation stones, one was found to have been stolen from the Church of St Cross, Middleton in August 1970. Not long after, the St Mary's Church, Kirkstall example was sold so that it too would not be stolen. The Bloomsbury building, University of London example is believed to still be in place (Royal Mint Museum, https://www.royalmintmuseum.org.uk/collection/coins/1933-penny/). Of the remaining four, one is in the British Museum, one is in the Royal Mint Museum and a further two are in private hands with unclear provenance (https://coins.ha.com/itm/great-britain/great-britain-george-v-penny-1933-ms63-brown-ngc-a/a/3105-32230.s).
The foundation stone examples were part of proof year sets that were placed in foundation stones (https://coins.ha.com/itm/great-britain/great-britain-george-v-penny-1933-ms63-brown-ngc-a/a/3105-32230.s). It is unclear if any other known examples are proof but institutional and private collection examples are circulation pieces.