Origin of 'bog-standard'

Today, while reading an interview of Camilla Pang in The Guardian of 28 Nov 2020, I found this in the article:

Deviations from that – whether through being a woman, or being neurodivergent or because of your race – hit a subconscious bias where some people think: “OK, how reliable is this person?”, and wait for something to go wrong because they’re not the bog-standard fit. [emphasis added]

While I have for a long time been aware of the term ‘bog-standard’ and what I surmised from context was its meaning, "down-to-earth, basic standard", I became curious about the origin — and also about what I felt was a slightly off-kilter use by Pang, but that’s neither here nor there. I "had recourse to" OED (to paraphrase Boswell about philosophy), and soon learned that the term was considered (a) recent and (b) of "uncertain origin". OED also confirmed my surmise about the meaning, although not my association of the term with ‘down-to-earth’. The first attestation given was from an ad in the April, 1962 issue of Motor Sport:

Bog standard Sprite, 1959, two owners.

The OED story about the term was similar to the story at The Phrase Finder, a source of "unequal reliability", and other sources summoned by invoking internet search engine demons (DuckDuckGo) repeated much the same story. (None of the sources, by the way, corroborated my association of the term with ‘down-to-earth’.)

The existing stories are summed up in the OED entry, which is new as of the 3rd Edition, March 2002:

Etymology: Origin uncertain; perhaps an alteration of box standard n. (although this is first attested later [1983]), after bog n.⁴ [a "toilet, a lavatory; the room or outhouse containing this"].
Differing theories of the origin of bog-standard have been proposed, but none proven. An immediate association with bog n.¹ seems unlikely on semantic grounds. The most commonly held view is that the transition from box to bog resulted from a mishearing or misunderstanding of box standard n.

Others have suggested a derivation < bog-wheel, former Cambridge slang for a bicycle, though ultimately also related to bog n.⁴….

Can anybody (who reads this and is willing, etc. etc. etc.) add anything substantial to the story of the ‘bog-standard’?