A rectangular concrete anchor block with a now cut steel loop embedded towards the front, antenna mast 8-facing side, providing an anchoring point for a guy rope, either for triatic straining or masthead straining. It is currently thought most likely to have been for providing additional outer masthead support.  All outer masts have the same arrangement, though all others are trapezoidal in form. The block measures 2.23m long by 1.7m wide, ranging in height from 0.38m at the southern side to only 0.18m at the northern. As for many concrete remains here, the block appears to have been cast in two parts; a 25cm-thick lower part (as determined at the southern, front side) and a 13cm-thick upper part. 

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

RCAHMW, 2023.