All struts that I have seen sold as Alpina struts had the tubes without the edge. Therefore, I have always believed that Alpina had its own designe on the tube. But It may be as you say W. It probably depends on the production year.The tube with edge, is probably most likely a newer design.
But why, changed the Bilstein the design on the tubes ... and when ?
The only one who could answer this, is probably Bilstein themselves