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It's a bit like a No 1 or even the original No7* which have one main standard held by two fixings and a brace, in this case the brace goes between the two standards rather than back down to the soleplate. * originally the No7 was like the No1 but they put it on an A frame standard and called it the 7A, the original No10 was the same and only became the 10A when put on teh A frame standard.
It's not till you see a no4 and a 10v together that you realise how much the size difference is.
and the 4 is rather smaller than the 5A or the Swan Jo