Callaway Steelhead X-14 Pro Series. Following the Big Bertha X-12s, came Callaway's Steelhead X-14s. These were the first iron to feature Callaway's Variable Face Thickness (VFT) Technology. VFT Technology allows Callaway's designers to adjust the thickness and raise or lower the centre of gravity for optimum trajectory with each iron in the set, giving a higher launch angle in the long irons and better control and increased backspin in the short irons and wedges for precise approach shots. Callaway Steelhead X-14s were the quickest selling irons in the history of golf - and they are still very popular. his Pro Series set features less offset than the normal model - allowing better golfers to work the ball more easily.