Below is a link to the small crown in the new website of the Royal Collection where it says the crown was worn by Queen Mary. It will be so exciting if "the book" includes a pic to support that statement.
In the biography by James Pope-Hennessy it quotes Queen Mary's dress book, for her first state opening of parliament but I have always treated the expression "Grandmama's crown" there as referring to Queen Victoria's regal circlet.
We have pics of Queen Mary wearing the regal circlet for that event.
"crown" and "circlet" are often used interchangeably.