Nigel has already provided the answer, but the lump does look symmetrical, and at first glance I thought tomwatts could be right in saying that it might be a trig point (seem to remember seeing a trig point on Waden Hill a few years ago as well).

Looks like Silbury is shown sketched from halfway along the Winterbourne path (from the Beckhampton Road). Trying to date the print... is that a bus or a wagon going along the Bath Road? If it's a bus it might be early 20th century in which case it would tie in with Nigel's date when he says, "The tunnel entrance was sealed in 1923 and the remaining mound that you see in this drawing was used to top up the hole in 1936."