This large, deep-bodied, stoneware bowl with reduced iron glaze and wax resist decoration has an impressed stamp in the form of a tricuspid. I have seen three pots now on eBay with this mark. From their style and provenance (the sellers are all based in Melbourne) they are likely to be the work of a potter active in Victoria in the 1970s and 1980s. Pots do travel but potters tend to exhibit and sell their work in their own state and sellers to source their listings locally.
Marks in the form of symbols can be hard to describe and I ended up using IP Australia’s Glossary of image descriptors to find a name for this one. The glossary is available as a PDF file about halfway down the Trademarks page on the IP Australia website. Another useful tool lets you find out how common a surname is in Australia. I’ve used this several times to infer the correct spelling of an inscribed name.
Potters’ marks are an important way – often the only way – of identifying a potter’s work. I doubt that many potters register their marks. I wonder how they go about choosing one and whether they are aware of other similar marks in use?
Postscript: A bowl dated 1981 with the tricuspid mark also has Robert Mair’s RM impressed mark. Images have been added to the Robert Mair entry on Identifying Australian Pottery.