The bolt pattern on H-D trike rear wheels is 4x4.25" or 4x108mm. This bolt pattern was used on some Ford and Dodge products. It is not exclusive to H-D. Even though those Diamond Smoothy wheels are the right width (15x6), I see three potential problems. One,those are advertised as "deep dish". Deep Dish means lots of negative offset. This won't work on a TriGlide or Freewheeler. Stock Harley trike rear wheels are 15x5.5" with 2.85" backspacing. That is an oddball sizing,to say the least. However,there is plenty of room to fit a wider wheel but the backspacing/offset has to be just right to fit your trikes correctly. A 6" wide wheel with 3"-3.25" backspace will fit great,as long as you stay with a 205 width tire.(stock size). These numbers still apply,if you want to increase the wheel diameter.
What's the second problem I noticed with the Diamond Smoothy wheels? I see no inner lip that would be needed to attach the Baby Moon hubcap. Third problem is if they are made of steel,instead of aluminum.