On the plug I have it's just a knob to help you grip it to pull it out. Now you mention it though, I do seem to recall seeing something like this with a plunger set inside the knob (i.e. grip it with first and second fingers either side of the knob and your thumb on the plunger in the middle).
Some of these old BS546 plugs (especially the huge
15A types) could really take a considerable amount of force to pull out of a socket. In an old building with crumbling plasterwork, it wasn't at all unusual to have the socket pull away from the wall slightly instead!
What was connected to these? Radios etc?
The 2-pin outlets were commonly used for table/bedside/standard lamps, radios (and TV in later years), electric blankets, and even vacuum cleaners. Even though the IEE specified 3-pin outlets for decades, it's amazing how many of the old 5A 2-pin outlets survived in old houses for so long, often co-existing with BS1363 outlets.