Correction - Arethusa and Galatea were the two initially fitted to carry two aircraft - apologies. However, Penelope was allocated two Seafoxes, according to Sturtivant.
Unhelpfully, Ballance et al The Squadrons and Units of the Fleet Air Arm (Tonbridge: Air-Britain, 2016) includes the following:
HMS GALATEA 713 Sqn K8588
HMS PENELOPE 713 Sqn K8588!
Sturtivant, however, only shows this aircraft allocated to Penelope from February 1938 (well before letter-number-letter codes came into force) until November 1939. Under the GALATEA section later in the book, he lists:
Seafox: (1 a/c 7.38 - 9.39): K8592 (no date); K8600 (8.38 - 10.38). (K8600 force-landed in Cyprus in October 1938 and was subsequently written off.)
Under the PENELOPE section he writes:
Seafox: (1 a/c 2.38 - 1.40): K8587 (from 2.38); K8588 (from 2.38).
This implies that Penelope was allocated two aircraft, perhaps with the second ashore until needed. (The ships of this class were intended to stow a second aircraft, with folded wings, aft of the funnel, for loading this onto the catapult, Arethusa and Penelope had a kingpost and derrick to starboard of the after funnel, but the arrangment was considered unworkable and the kingpost and derrick were removed from these two ships and omitted in the other pair.)
I can think of four possibilities. One is that the aircraft, for some reason, was being cross-decked. The second is that the same photo was available for sale on every ship that operated the Seafox and captions... well, who cares about that sort of detail? Perhaps not the person who captioned it, but he could simply have got it wrong. Or, perhaps less likely but not impossible, Sturtivant and Cronin were working from a piece of duff information.
I suspect the second.
A question for our Fleet Air Arm experts. I just bought a photo of a Fairey Seafox K8588 with the code H9B painted in profile on her starboard float. Ray Sturtivant & Dick Cronin's book Fleet Air Arm Aircraft, Units and Ships 1920 to 1939 lists this aircraft as being assigned to HMS Penelope but, the photo I bought is from a crewman of HMS Galatea and has this aircraft captioned as being launched by Galatea.