It wasn't until I noticed that in both of these pics there is a fly ash hopper barge passing downriver in the background, which suggests to me that the views were taken on the same occasion.
Perhaps our resident Naval experts can come up with some details

There is only one snag in your theory that I can see ARTFUL was an A Class which had a completely different profile to the 2 boats alongside together which are either Oberon-or-Porpoise Class I never could tell the difference. I'm sure Brian will know as I believe they could be his photographs.
Dave

Round about June 1965 Porpoise and Sealion berthed alongside together. I'll confirm the date ASAP.
Does that sound more like it?

June 1965 sounds quite reasonable to me. The pics are not Brian's. All of the colour images that I post on NEM are from colour slides taken by myself

The upper picture of Artful might have been taken during the same visit as this picture, which was taken on 14 October 1967.

To put a date on the two "O" boats this is OTUS and ONYX taken on 14 March 1968. ONYX is now awaiting her fate on the Clyde.

To tie in with the Artful views, I took this one on 13 October 1967.