Transport of the Stonehenge Bluestones: Testing the Glacial Hypothesis by JD SCOURSE · 1997 http://publications.thebritishacademy.ac.uk/pubs/proc/files/92p271.pdf highlights this:
In Hawley's 6th Report Jan 1926 Vol V1 No.1 The Antiquaries Journal which is of his 1924 excavations he describes finding a foreign stone and also a photo of the excavation.
I think the stone pictured in the middle of the very left hand side of the excavations around stone 8, judging by the scale, is about the right depth and size to be the foreign stone he mentions. This provides the original context for the stone. (Click to embiggen)