Indeed. So which one would that have been? . . .
The signalbox shown on that map was Barton Moss Junction. That was pretty much on the eastern extremity of Chat Moss and around where the M60 Motorway now crosses the railway.
Barton Moss Jn was most likely a late 19th Century addition, as the junction in question was where the double track coming east across Chat Moss from Parkside used to split into four lines (fast & slow) on the approach to Patricroft. Barton Moss Jn was just over a mile west of Patricroft Station. It was a different location to the two earlier Barton Moss stations located well within Chat Moss.
It seems Barton Moss Jn signalbox actually survived until late into the 20th Century as an intermediate block post between Astley and Eccles boxes, and long after its function as a Fast line / Slow line convergence had ceased.
The signal box was damaged by fire on 19 September 1990 and closed on 6 October 1990 as a result.
Another signalbox located in the wilds of Chat Moss was
Astley Green Sidings. This was 2 miles 957 yards west of Barton Moss Jn and 712 yards east of Astley level crossing (and the former Astley Station).
Astley Green Sidings controlled a triangular connection to colliery exchange sidings on the north side of the main line. From the Astley Moss exchange sidings, an NCB line ran about 1¾ miles north to Astley Green Colliery. This line was built between 1908 and 1912, when the new colliery was being sunk. As it had to be built across the same characteristic boggy terrain that Stephenson encountered in pioneering days, the mineral line was built in the same way as the original L&MR (i.e. floated on brushwood and timber sunk into the bog and covered with colliery waste).
See
this extract of a 25-inch OS map from 1928 at the
National Library of Scotland.
Astley Green Colliery closed in 1970 (it's now a coal mining museum) and there are plenty of photos of NCB tank engines with 16-ton mineral wagons storming along the line in its later days. However, I've not yet found any photos taken at the BR end of the branch at Astley Green Sidings.
Presumably BR's Astley Green Sidings 'box was abolished in the early 1970s after the colliery had closed, due to its relative proximity to Astley Station ‘box (less than half a mile).