Here's the best photo of the Gridiron I've found so far: Item PG/3596 in the Isle of Man Photographic Archives.
The photo is dated 14 May 1946 and by amazing coincidence, looking at the boat's configuration and position, I think it may be exactly the same boat that's in the 1946 aerial photo from Sunday's wander.
Edit: later I found this colour picture of the Gridiron on Jem Southam’s Instagram feed, which was a lovely surprise!
GPS Coordinates:
51.44882, -2.62391
Location: Hotwells
