A concrete base for one of the 400-foot high tubular steel mast supports, mast 5, for the 1914 wireless antenna. It has 16 steel threaded bolts of 2 inch (imperial) diameter, protruding 11.5cm (4 ¼” as built), set in a ring 1.8m (6 feet as built) in diameter. A faint impression of the outer tube base flange ring is visible, being 2.3m (7ft 6 inches as built) in diameter. 

Source: Rowlands, J., Marconi's Carnarvon Station 1912-1939: a journey into early commercial wireless in north Wales' Second Edition, 2023.

RCAHMW, 2023.