In a story by Denton Welch I came across this fragment:
"... and playing with the scent bottles, matchboxes and engagement tablets of old cracked ivory-the cracks were black, like my nails when I was sent to scrub them". [Denton Welch, 'The Trout Stream']
What are "engagement tablets"? I can guess it's something small, ivory-colored, with cracks. Engagement ring boxes, perhaps? :)