Originally Posted by kdelong .
You're in luck. There is not much to tell about the 105 groupset prior to 1990. The 105 groupset was born, so to speak, in 1986 when Shimano wheeled out a couple of new innovations, namely their Shimano Indexed Shifting(SIS) technology and their Shimano Linear Response(SLR) brake technology. They used their existing Exage 500EX groupset and added the SIS and SLR features to it and renamed it 105. At the same time, they discontinued a whole host of former groupsets like the 200GS, 300GS, 400GS, 500EX, and Exage. They re-introduced these groupsets under new names with added SIS and SLR features over the next several years. The last name change was the Shimano 600 AX groupset which was changed to 600/Ultegra in 1993 and then became just Ultegra in 1996.The Dura Ace name never changed but the groupset went through its own re-design so that older pre-1990 parts are generally non-compatible with the newer post 1990 parts.
Since its introduction as a groupset to showcase Shimano's new technology of 1986, the 105 has taken its place as the most affordable high end groupset made by Shimano. Although it typically follows Dura Ace by a year in development, it does benefit from advances that are first introduced on the DA groupset.