I was speaking to Glen today, and she mentioned this thread. In her book 'A Century of Carnival Glass' (isbn 0-7643-1209-X), there is a small entry on Page 51 for 'Jackson Brothers Ltd, Knottingley'. The jar in this thread is one of theirs. She has the same one in smoky grey with the RD number, and hers has the JBK moulded trademark.
She's never seen one of these jars with a label, but they were designed for a cosmetic company (something like Jean Poitou?) who used these jars for their products.
Many thanks to Glen for passing this information on.
