I reported - http://www.sarsen.org/2012/01/three-original-standing-stones-of.html - that David Field had floated the idea that there was a central mound in Stonehenge just to the south of Stones 53 and 54 that might have predated the stone circle. It lines up with the North and South Barrows, in the post I used this diagram - Stonehenge contours at 0.075m intervals - © A. Johnson 2008 - and circled the three "mounds". David thought that they may have originally held three stones.
I reported that there was no records of any excavations in the central mound area to take this theory further but rereading Gowland's report on his excavations in 1901 I noticed he had dug a hole, marked F on the plan on the edge of the area. The hole was dug to take a supporting strut for Stone 56 as he straightened it.
The details of his excavation there are not in the online version of his report - http://www.archive.org/stream/wiltshirearchaeo33arch#page/60/mode/2up - because they are in the appendix. But in the Archaeologia printed report they are, and it reads:
Excavation F. 5 feet long, 2 feet 10 inches wide, and 3 feet 3 inches deep.
This excavation was made at the side of the south-west pier, No. 54, of the south trilithon. .
The chalk rock was reached at the south-west side of the hole, about 3 feet from the surface.
About 1 foot below the turf, and to a distance of about two feet from the monolith, the entire space was closely packed with chippings of all the stones, together with two or three large lumps of sarsen, one of which measured 1 foot 6 inches by 1 foot 2 inches by 1 foot.
All these were more or less firmly cemented together with a calcareous concretion.
Splinters and pieces of sarsen hammerstones preponderated over those of ordinary sarsen, and the latter was found in much less quantity than the “bluestones;” but besides the clippings, however, several large irregular lumps of sarsen were also dug up, as stated above. As regards the “bluestones,” there was much more porphyrite than diabase, and least “fissile rock” and argillaceous sandstone.
Seven small fragments of ancient pottery were found about 1 foot 6 inches below the surface, and a few pieces of micaceous sandstone.
David Field suggests that this hole F was into the stonehole for stone 54 rather than into the mound which seems to be correct. Cleal's plan is slightly out which is misleading.but Gowland's original plan shows the excavation was tight up to 54.
The two plans compared (not exactly to the same scale)
And from the NMR we have this photo
UPDATE - I have found a postcard which shows Gowland's excavation F as a parchmark:
Click any picture to embiggen