Cleal's Stonehenge in Its Landscape: 20th Century Excavations (English Heritage Archaeological Report) seems to be mistaken ('Quandoque bonus dormitat Homerus') as to the stonehole of the fallen upright of the Great Trilithon Stone 55.
Here is the passage from Cleal and the Figure 12 referred to from Gowland's report.
The vertical scale in Gowland's diagram is in 6 inch increments (150mm).
Click picture to enlarge.
Trilithon III, the ‘Great Trilithon’
Trilithon Stone 55, WA 4143: Gowland excavated be
neath one part of this stone (55a) which is fallen and broken
into two parts (excavation Q Gowland 1902, 52—4, figs 7 and
12) (Plate 5.3). He did not find a clear well-cut hole but only
a ‘shallow cavity’ and he postulated that the stone had stood
in the cavity, propped and wedged by sarsen stones which he
found lying under the fallen stone. The ‘cavity’ as it shows in
section is barely 2ft (0.6 1m) deep (op cit, fig 12) and it seems
unlikely that such a large stone could have stood in it. Without
full excavation of the area under 55a, however, Gowland’s
observations and interpretation cannot be tested. The section
which shows the ‘cavity’ (je Gowland 1902, fig 12) also shows
a much clearer and deeper cut which looks similar to the
stonehole for Stone 56. This, however, if it was the hole for 55,
would place that stone too far into the interior to line up with
Stone 56. Gowland does not explicitly describe this cut but he
may be referring to it when he describes how two of the blocks
of sarsen which he considered to have been supports for Stone
55 when it was standing ‘had the ground entirely cut away from...
Cleal says that the hole is barely 2 feet deep (ie four six inch increments). This is the measurement from the underside of the fallen stone to the bottom of the hole. But the stone is sunk into the ground. On the left can be seen the ground surface level. One can only estimate the the ground surface when the stone was upright but it is probably close to the level marked. This would give a hole depth of about 3'6" or 7 of Gowland's marks. The bulbous base of 55 is also about 3' 6" from the worked flat side to its base indicating that this is the depth it probably was in the ground. Gowland's hole fits perfectly.
The cut in front may be part of the very ancient hole that was dug in front of the Trilithon for reasons unknown. It is also not clear how deep and far under Stone 55a Gowland excavated and if the illustrated cut extends under the stone or is the end view of the Western edge.
For more on the original position of Stone 55 see http://www.sarsen.org/2012/06/original-position-of-stone-55.html