A size in inches, which would be diameter, height or length of the item, sometimes appears on jasper and basalt items from around 1840.Eighteenth century cameos and intaglios sometimes have a number impressed on the back that refers to the catalogue, and can be matched with the catalogues reprinted in the references. According to Reilly all black basalt made from 1769-1780 was either unmarked, or had this mark; however this is by no means certain; see the following paragraph.There is evidence to be found in Wedgwood's correspondence that the usefulwares factory was still making basalt tea sets during the Wedgwood & Bentley period (1769-1780) and marking it with this mark.These are not date marks and, with a few notable exceptions, have no meaning to us now.A three letter date code is sometimes found and starts in 1860 but is rarely found on jasper or basalt items; it absence is not indicative of any date.
This appears to be a potter's mark, and belongs to the period 1795-1850; perhaps a little later. This mark belongs to the usefulwares factory before 1780. Apart from basalt, the Wedgwood & Bentley mark belongs to ornamental wares only; useful wares had : Before 1800; most likely 1780-1790.Dating Wedgwood can sometime be very difficult as apart from the Trademark there are also in some cases letters that accompany the marks to give a more accurate manufacture date and most old pieces have this second mark.To better date a particular piece collectors will often also refer to this marking.These often have a catalogue number beginning BB, and usually have Bert Bentley's mark: a zero on a slope or on its side.