This is a real noodle-baker. I'll try and summarise what we have.
John Lewis wrote an article in Great Western Journal in 2007 on GW 16ft Vans, which had this in a section on DC.III brakes:
This matches with what we've been looking at, as there are several preserved V14s where the V is in the centre of the vehicle:
101720 at Didcot and 101961/103592 at the SVR are examples of this.
In this photo it is below the central body support stanchion (which is shown as being dead-centre on drawing 78302). This is mid-restoration and hasn't had the vac cylinder refitted yet, it is down as a V14.
This tallies with some of the works drawings we have for underframes where both the unfitted and fitted versions have the V marked as 'centre line of vehicle'.
Here is a comparison to show just how offset the V was.
Note that in these two pics, the van with the offset V has the slim buffers too. Unsure if there may have been a changeover year we can pin this down to, any ideas?