... center line of the chair, which is backed up by a footnote to Table 13 "The 'A' Chair comes under the nose of the crossing, the centre of the chair being 4" from the blunt nose". The main body of the built up chair is 8" wide, with an 3" extension on the front. From the close up pictures I've got (either Scarborough Station, or Crewe Gresty Lane) of track, the front of the built up chair coincides with the front of the timber: 499_121147_100000000.png This works out at the blunt nose being 3" from the C/L of the A timber. Although there's an inch to play with (12" wide timber- 11" wide chair), so really it's between 2 and 3". For solid cast iron 'A' chairs (which I've never seen, the blunt nose is 2.5" from centre line of the keyed jaw, and as the chair is 12" wide, there's only one place this can go on a 12" timber, so that ...