(click to embiggen)
An early postcard (1930?) shows Gowland's prop holes as pale parchmarks and also the stoneholes 17,18 and 19 as faint marks (17 is the clearest). They run in an arc from the upright stone on the left, stone 16, round in front of the group dressed in black towards the upright stone at the top middle of the picture. Stonehole 20 is mainly under the fallen stone with the black mark on it.
For details of their discovery this summer see here