On: Bog Standard (2005)

[InsideOutSanta]

This link has one inconclusive bit of evidence that it might be derived from "box-standard": https://www.worldwidewords.org/qa-bog1.html

  In February 1983, [Sir Clive Sinclair] said in
  an interview with the magazine Computerworld:
  “Luckily, we cannot foresee the day when a
  computer becomes just a standard box. There
  will be box-standard machines along the road,
  but we do not simply have to make those”.

[nhinck3 → InsideOutSanta]

Google books pulls out plenty of uses in the 60s referring to unmodified cars and car parts.