Why can't I use the stud length 1 with the flush grommets of the D4002-series?
This isn't possible due to the size of the flush grommets.
these are a bit larger than the plus flush grommet, but stick more inside the panel.
So when a stud with stud length 1, for example D4002-1BP is inserted in the grommet, the stud won't be able to reach the receptacle.
Therefore, if you wish to use the flush grommet and depending on which receptacle you wish to use, need at least 1.30mm total panel thickness, with a stud length of 2.