The first Dura Ace Hyperglide compatible hub. Was produced to be backwards compatible with UniGlide - note the UG outer threads on the freehub body. The lock nuts are tapered to facilitate installing the 130mm hub in a 126mm spacing frame.

FH-7400 6/7 speed UniGlide (reduced diameter 31.8mm UG thread to allow 12T sprocket, 126mm spaced rear)
FH-7402 8speed UG (reduced diameter UG thread, 130mm spaced rear)
FH-7403 8speed UG standard size UG external thread, full length splines, plus HyperGlide spline and HG internal lockring thread.(130mm spaced rear)

The 7400 series share a unique attachment method. Instead of the normal bolt there is a threaded protrusion built into the freehub, with it's own unique extractor tool.

Shimano's top of the line race quality component group in the mid 80's through the early 90's. The 7400 series Dura-Ace group saw several interations including the introduction of 6 then 7 and finally 8 speed SIS shifting. Eventually upgraded with 7410 parts before finally being completely replaced with 7700.

Dura-Ace was developed to give the professional racing cyclist a fraction-of-a-second advantage that can spell the difference between victory and defeat. Advanced functions like 8 speed SIS, Super SLR brakes and Shimano Total Integration provide the most refined feeling of control available in a group today. Ultra high-precision mechanisms, quality materials and rugged construction assure that Dura-Ace takes all the punishment that pro-level road racing can dish out. (1991 Dealer Manual).