Hey malt_shovel,
I don't have an answer for you but have seen the same thing on mine. No burrs as yet though.
The lateral bines are probably 400mm long up at the extremities, but down lower on the plant they are much much shorter, some even just tiny leaves.
A lot of guys use a collapsable trellis system, I have done a variation myself. Vertical lines, then diagonal down to a fence, when they reach the fence I will cut the line and drop the plant back to the base of the original line and I am hoping the bine will climb back up itself.
Mine are first year plants too, doing well.
I am looking forward to a few flowers soon !
UB