While the hole sizes for your ONDU's have been optimised some users like to experiment with their pinhole size. While a smaller pinhole size might yield in a sharper image it will produce slightly more vigneting on the edges of the picture and a larger image would do the opposite. The pinhole is nestled and protected in a black anodised aluminium housing that is universal to all ONDU MK II cameras. These replacements will not work on the first generation of ONDUs.
Exchanging the pinhole is relatively simple, just push it in form the outside and replace with the new one. If you need more guidance on that part we'll gladly assist you.