In 2010, whilst making a George II reading table, I turned a wooden thumb screw to lock the adjustable column at various heights (fig. 1).
Fig. 1. Turned apple thumb screw.
In part five of the table’s construction, I mentioned the style of screw was fairly typical of the decoration on wooden and brass screws used on furniture and architectural joinery throughout the Georgian period. I recently turned up an image of a circa 1760 table with just such a thumb screw (fig. 2).
Fig. 2. George III mahogany reading table with turned wooden thumb screw.
Jack Plane.
Pegs and 'Tails 
Function ? The “dutch” drop seems to be able to engage the centre column securely, unless its to hold it temporarily….
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The spring-loaded Dutch drop (a frequently seen fitting on these sorts of tables) engages the perforated brass strip on the column and provides positive location. My guess is that the column became a loose fit in the pillar and the thumb screw was added to counter any tendency for the table top to sway from side to side.
JP
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Thought it may have been a ring in, nice butterflies, poss later ?
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Yes, a much later repair I would say.
JP
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A useful detail that is just visible – the upper round collar is held together by a pair of double-dovetailed ‘dutchmen’ aka butterfly keys. No nasty screws to rust or strip, no ugly end grain on wooden dowels, no worry about glue failure either.
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