Update: Rob has identified this as Pillbox 063]
Although the wartime maps don't show the location of pillbox 061, we're guessing this is where it was.
The red marker shows the approximate location of the northern tip of the western arm of the old vehicular ferry pier. You can see the pillbox on this photo of the ferry pier in the 1950s:
Then based on the positions of pillboxes 59 and 62, and what we guess was pillbox 60, then this would have been #61.
If you have any information about whether this was or wasn't #61, we'd love to hear from you!