The MF-7400 was a 6 speed, SIS freewheel. The MF-7401 was the 7 speed version, as stated by Sheldon. There should be a date code on the outer cone/cover of the freewheel body, though it may not prove much as the two freewheels were manufactured concurrently for a while. It sounds like someone tried some creative mxing of cogs, spacers and freewheels bodies.
As stated by the previous poster, the original Dura-Ace STI Levers were 8 speed. The levers are designated ST-7400 and if you peel back the outside of the hood, at the handlebar end, you will see the marking. The date code is on the opposite side of the lever body and a bit higher up, so you will have to peel back the hood a bit more to see it.