All those GPM bits (DOs, stay bridges) and the slotted BB shell plus the BB threading certainly says "Italian" but if your seatpost size can be confirmed with a better tool than calipers (which unfortunately are not the best for such an internal tube that can become distorted) POSSIBLE this is an oddball that used METRIC tubing. Very rare for an Italian builder but not unknown and would explain if this is actually 26.4 or 26.6.The stay caps ring a faint bell and may be a distinctive clue, can you add some close-ups to your "album"?