If you mean the pre 1986 Nuovo Record version with the rounded cutouts, then the rear dustcap is the same for all versions of the Nuovo Record rear hub (LF, SF, road, track), part number 740.
I just measured my NR s/f rear road hub and it's 27.5mm. That's measured with the dustcap inserted in the hub, you would need the dustcap by itself to get a proper measurement.