The IGC rules changed between 2015 and 2016 regarding the wing contest number requirements.

2015 IGC Rule 4.3.1
4.3.1 The contest numbers, as validated by the Organisers, shall be displayed:
a. On the underside of the right wing, approximately 2.5 m from the centreline of
the sailplane with the top of the figures or letters towards the wing leading
edge. The height of the letters or figures should be not less than
approximately 80% of the wing chord.
b. On both sides of the tail fin and/or rudder. These must be at least 30 cm
high.
c. On the glider trailer and crew car.

2016 IGC Rule
4.3.1 The contest numbers, as validated by the Organisers, shall be displayed:
a. On both sides of the tail fin and/or rudder. These should be at least 30 cm
high.
b. On the glider trailer and crew car.

Link to the the IGC 2017 Rules
https://www.fai.org/sites/default/files/sc3a_2017b.pdf

Jamie Shore