A parcel, Preload front fork adjusters, has arrived today from Taiwan. I purchased these at ebay shop, simonchantw_gmail who has listed various Bonneville parts on ebay. Most Bonneville users have noticed that similar parts have been around at other Bonneville custom parts shops (such as British Custom and newbonneville). I have noticed that those custom parts from Taiwan are all excellent products with high accuracies.
The preload front fork adjusters, also available in black anodized finish, seems to be excellent product at a glance. By replacing the original fork cap with this, you can now adjust the preload on the front fork suspension (by up to 12mm). I have not installed yet, but looking forward to see the outcome.
PS: Installed today on my Bonneville.
Comparison between the original fork cap and fork adjuster. You may notice the more preload at onset by the thickness of the bottom plate (gold color) when the adjuster is used.
If the third-party handle bar is used, the bolt sticking out from the adjuster may come to contact with the bar. Shown here is the Norman Hyde M-bar.