I measured mine with my mic. NDS bearing surface was 0.5894" and DS was 0.5891"... so 14.95~14.97mm
When the axle is slipped through the bearings there is noticeable play. Once the DS locknut is tightened the play is eliminated. On my wheel the DS locknut tightens against a spacer which slips over the axle and pushes against the bearings inner race.
I posted a detailed account here once on how to use Miche Primato hub hardware on a 900. The Primato uses a 10mm threaded solid axle with "cones" and lock nuts for 15mm ID sealed bearings. It allows you to set the preload and lock the nuts against the cone; doing away with the centre sleeve. It's probably less rigid than the stock track axle.
I have a lathe and may be able to make a replacement if you don't mind a long lead time.