Let me guess the longer one is older?
The first several runs had barrels just over 17" they designed the rifle with a 16.25" barrel and then after the fact added the threaded portion, they never changed the length in their literature.
Later they made the special run with the 18.5", and it was brought to light that the barrels on the others were not really 16". So the newer ones listed as 16" are actually 16" not the 17"