Just checked, and it was 12 aircraft, converted from HC2, and on detachment with 22 Sqn at the bases around the UK - Manston, Valley, Boulmer and Leuchars.
As mentioned, outwardly they were basically the same as the HC2, apart form the overall yellow paint scheme, and a larger winch, with 300 ft cable.
Internally, the main alteration was a 'wet' floor, basically a waterproof floor and covering to prevent corrosion, plus the mounting of specific equipment such as First Aid kits, rescue stretcher, strop slings, oxygen resucitation kits etc, as well as the extra navaids, the antennae for which can be seen externally on lower fuselage and nose.
The flotation 'cans' were later added to the main gear wheel hubs, and these contained inflatable spheres, for use in emergency ditching, activated by explosive charge on contact with water.
Various spot/search lights could be fitted, beneath the fuselage and on the starboard main gear leg, the installation of which varied. From memory, as far as I know the black colour on the starboard gear leg, which was added later in service life, was an anti-slip material, and also acted as anti-glare when the gear leg spotlight was used.
I'm sure I have the serial numbers somewhere, but haven't found the darned list yet !