Army Form B.2512 is the attestation form that was introduced in December 1915 in response to the creation of the Derby Scheme which it references in paragraph 11:
This form was to be used for wartime enlistments only with the most noticeable addition being the box, top right, in which the man's Derby Scheme group number was to be written. In this particular example the man fell into Group 33 and this number also appears on the top left of his form, presumably written there as a filing aid at the time of his attestation.
You can read more about the Derby Scheme on The Long, Long Trail website. All document images reproduced on this post are Crown Copyright, The National Archives. Service and Pension records can be searched and downloaded from Ancestry and Findmypast. Both companies offer FREE 14-day trials.