https://www.bda.org/library/history/PublishingImages/Pages/index/Swanson_BDJ_255.pdf I now realise that my allotment digging may have unearthed something quite a bit more unusual than i'd initially imagined, and in case anyone hasn't seen this - but i can tell that pot lid enthusiasts are obviously very well informed, so it's probably unlikely! - i thought the attached article might be of interest. Pot lid for sale here is the same as example pictured in Fig 2 on page 2.