Unusual English early 19th century ribbed bone set, 3.9" kings

This set is unusual in that it is made of bone - except for small discs of ivory inserted in each of the "barrel" pieces (kings, queens and rooks), which the stems screw into (in the case of the rooks, the ivory discs form the plus for top and bottom). I have not seen this in a bone set before. There is some speculation that this was an efficient way for a busy workshop to use up scrap pieces of ivory (though why they didn't save those scrap pieces for their ivory sets, I don't know!). Very solid chunky feel to the pieces, which makes me think this is early 19th century. Knights are thin, as is to be expected being carved from bone.