The A874 was a short road linking the A811 and the A813 near Balloch at the southern end of Loch Lomond.
The number was probably allocated c. 1926, as the numbers preceding and succeeding it can be dated fairly accurately. However, the only known record of this number are from the 1930s and 1940s, including the Ordnance Survey Ten Mile Road Maps of 1936-8. The question is therefore whether the road was missed off the 1932 map owing to its short length, or whether this is actually the second A874, allocated around 1935. The latter is most likely, as during the 1920s this area was rural, and the road has the feel of a 1930s build.
The A874 number applied to Carrochan Road in Balloch, a road which appears to have been new-build in the later 1920s, and is now the route of the A813. This route cut the corner from A813 to A811 southbound (remembering that these routes were swapped in the 1970s), and was presumably built and classified in an attempt to provide an alternative route to the congested A82. Later, but before the A811 and A813 swapped in the 1970s, the former route of the A811 via Shandon Brae became unclassified due to new housing in the area. At this time, the southern half of the A874 became the mainline of the A811, whilst the northern section became part of the A813, meaning that the two routes cannoned at the roundabout.