I've reset the oln distance to 130mm.
I started using the threaded bolt with washers & nuts method, and went to 132mm ish by opening to ~150mm. Discovered that the DT wheels /Miche hubs still didn't go into the dropouts easily - then realised that the width of the dropout 'slot' must be a bit too narrow on this RRS 501 frame. The miche hubs have an unthreaded hub 'shoulder' which must be slightly too wide for the frame. Which possibly explains which I couldn't spring the frame 2mm each side to get the Miche/DT wheels in.
Some std Shimano wheels fit OK. So at this point I started pressing and pulling the frame, which is surprisingly easy to do - to get the right oln distance which is now at 130mm and SeatT equidistant from string.
Not checked to see if dropouts parallel.
Any thoughts? I guess I'll just stick to the Shimano wheels, as widening the dropout 'slot' wouldn't be easy/doable AFAICS.