Excellent find Mike, shall enjoy these.

As for the rocking twin phenomena it wasn't just under the purview of British iron. As late as the nineties Japan was still knocking out rocking twins in their 4-stroke range, especially the small bikes and the vibrations these beasts produced would rival anything from Small Heath.

I went through a slew of Japanese machinery back then..

HONDA.
CB/175/250/350/360
CJ250

YAMHAHA

GX250

There are a few others lost in the depths of my memory but the point still stands that these buggers vibrated just as much as UK iron and the only twin I had that was smooth as silk was the CB400T "Dream" which relied on power sapping balance shafts to iron out the grumbles.

A lovely chunky bike a born cruiser and very economical and sold over in your neck of the woods in automatic guise as the Honda Hawke.