Tashi, I'm thinking the 27.4mm post is a red herring. Isn't doesn't take much effort to insert these into a standard butted tubeset. About the only tubeset from this era that would have used that size post is MAX and it's obviously not that tubeset.
Those Gipiemme dropouts were typically found on the mid-range, Italian manufactured frames. We know it's at least 1991 by the Superset II tube configuration and those dropouts don't match what was on the Japanese manufactured frames. In 1993, in addition to the SBX tubeset, there was also a Superset II version of Columbus Cromor for the mid-range models. Pending further evidence, I feel this is the best candidate.
Things to can check into, in the future, include:
1. Scrub the bottom, inside of the steerer tube with an old, stiff toothbrush and some solvent, to determine if there are splines, their number and pattern.
2. Remove the bottom bracket and determine the threading, which should be listed on it. BTW, a cartridge bottom bracket was not offered for Dura-Ace in 1991, providing further evidence that it's not an original groupset.
3. With the bottom bracket removed, check the inside of the tubes for ridges.