although this doesn't really have anything to do with Roy's custard cups, I thought the following comment from the people at Kew quite relevant to our occasional fruitless searches for items which should appear in the Blue Book, but are missing. Glass items which should be under the CLASS III
From 1842 to 1883 the representations were filed according to their 'material classification' (what
they were made of - see the classification table below). The clerks sometimes placed items in the
wrong material class, so that a book binding may be mis-filed as a carpet (as was BT 50/52
design 38781).