been at 2: interestingly (yes!) re 6, I did know the definition, having known a few of them in my time, but was held up by the clever surface which made me think for too long that it wasn’t an SI clue but a charade with a one-word definition.
It would be interesting to know what others’ FOI of the SIs was - without the benefit of your hint mine was 18, which may say something subliminal about my preference as between unclued 18s and unclued 27s!