Original smooth twist is
Smooth bore till the last 2 inches or so with yes polygon rifling thats pressed on from the outside causing a choked barrel. If anyone has a source of Fredrik borrowing this idea from anyone please cite it. If you don't know what you're talking about maybe you should not try to imply that this idea is stolen.
Glem has it right. Others are guessing. But I think I see the "red herring".
The FX ST 5 lands rifling is pressed in from the outside of the barrel as grooves which then raise rhe lands in the last couple of inches on the inside of a smooth bore blank. The result is a land contour somewhat resembling polygonal but isn't. In the later STX barrels the rifling is full barrel length.
There is no resemblance to Whitworth.
However when the pellet strikes that last couple of inches of the original ST rifling, the pellets strip through and emerge with a much lower spin rate than the actual rifling. In my original study JSB pelllets spun at 1:64 " from the 1:16" twist rifling. (Note that different weight pellets result in different spin rates from the same barrel because of differences in impact momentum when they strip through the rifling).
Now therein possibly lies the source of the errors in thinking that ST is a rehash of the old Whitworth.
Because the pellets have stripped through the ST rifling they do not emerge with defined grooves and lands but basically with just 5 flats. My original pics of pellets, caught in teased out Merino sheep wool, showed this perfectly. Thus the pellet does appear somewhat like a Whitworth bullet.
This pellet profile was not intended and when I made Fredrik aware of these dynamics he was most surprised and spent a weekend duplicating my study model and results. Ultimately the result has been the STX revision with full length rifling, and with the possibility of accurately producing liners which can induce any spin rate desired to optimise individual pellet accuracy. ... My FX STX Crown is super accurate with all three versions of the FX King .25 cal pellets. It has a 1:28" twist rate . ... and the rabbits in the vicinity of our farm are most displeased as they have lost over 400 of their mob to it during 2018.
I hope 2019 will be kind to you all.
My best regards, Harry Downunder, where it is presently a pleasant 84 degrees F.