This number is unknown, and probably non-existent. Shape book 'B' steps straight from 87 to 89 with nothing between. The omission of 88 is thought to be just a mistake made while setting out numbered panels in the notebook. Shape book 'A', which was later copied from book 'B', does have a numbered panel but it is left blank.
No example is recorded in the image database which is not surprising, but if you happen to have one, please contact us!