I had the repair shop check the swingweights on a 913F 15* and 913Fd 13.5*. The former has a 72g Diamana and stock headweight. The latter has a 70g Diamana x5ct with an 11g headweight. Both are 42.5" and have Winn Excel RF oversize grips. The swingweight of the 913F was D4 and that of the 913Fd was D2. Is there really a 6 gram difference in weight between the 913F and 913Fd heads?