All four back I suppose. The
concrete starting area was tried in the post war years but quickly
fell out of favour. By the looks of this photo the concrete
must have provided lots of grip for the back wheel and would have
stamped out (pun intended), all the riders digging with their feet
at the starting grid.
Vincent better known for their roadster
1000cc V Twins unveiled their 500cc Speedway machine in 1948.
Alas, it never caught on against the well establish JAPs