Hi Wade.The B33 finished in 1960 (1959 for B31). Therefore they did not have arrow head stays, these were only on A7/A10 models.
That rear mudguard you have looks right for 1956-60. The flat in the beading on the edge is there for the flattened tube stays to pass over.
When buying stays be carefull with pattern items, a lot of them are made with the flat central to the tube. The rear edge of the flat should
be parralel with rear side of the tube.
regards, Pat A.