Go into your 'User CP' menu and select 'Edit Profile.' The 2nd or 'Optional Information' box on that page allows you to enter a 'Custom User Title.' vajeep has pointed out you have chosen to place your trailer type there.
If you delete the info from that line and go to the 3rd major 'Additional Information' box. You don't really have to enter your VIN I believe. But in the Trailer 1 area there is a window with a scrolling option -- '69 31' Sovereign
is one of the choices. This way both your rivets and your trailer type will show up in your avatar box when you post messages.