What is this runway marking on Kirkwall Airport's runway?
That's the aiming point mark, UK style. The UK AIP GEN 1.7 (2018-8) notes the following reasons for the different marking:


  
  The shape of the marking means that 1/3rd of it is outside the centre 3rd of the runway and is therefore less prone to rubber contamination.
  The marking is more easily identifiable as it differs from the TDZ markings.
  It provides enhanced visual cues for the angle of approach.
  





(Source: CAP 637: Visual Aids Handbook, May 2007)