Greetings,
All the Star Dies I have encountered have the following characteristics...
The #18 has always had a Roll Crimp and the top of the die is threaded for a seating punch.
The #18T generally is about 2.0" long and has no threaded hole at the top. Often it is the bullet diameter clearance all the way through.
I have deviated from the above by machining a Roll Crimp die with no threads for the seating stem. I try to mark the end of the die to distinguish this variation.
In the past, if I wanted to roll crimp in the last station, I would order a spare #18 die from Star.
By the way, the #18T style dies are easier to clean than the #18 dies. I just run a Bore Snake © through the die once and a while.
Cheers,
Dave