Two further bits of information, one of which looks as if it might not only confirm your theory but also reconcile it with zero being at the junction for the Bolton line. The first is from the Railway Performance Society, whose distance-chart for the Victoria-Rochdale line notes that the present zero marker is wrongly positioned to the west of the station footbridge and that in fact the present buffer tops of platforms 1 and 2 are at minus 0m01ch. The second is a screen-grab of a1:500 OS map of 1891 (taken from
www.old-maps.co.uk, the condition of their free service being no commercial use—and the fee for their paid service allowing greater magnification a massive £9.99 a month—how lucky we all are to have the NLS make its collection available even in England!). This shews the station after the first major enlargements but before the last group of bays and the new buildings were provided, so shews very clearly the relationship of the first of the bays and the junction of the Bolton line to the old buildings. Unfortunately for this query, whilst all signalposts and lamp-posts are marked, mileposts are not marked! But if one looks at the Bolton-line junction, it is clearly to the east of the centre-line of the old building and so well and truly on L&Y territory. But also there is a very curious symbol just to the north of it, on or by a lamp-post, of a rectangle made up of two squares, a lower bacl one and an upper white one. I haven't come across one like this before. It's also very nearly in line with the endof platform 4, which looks as if it could be one of the original bays. Could this be the zero-point?