I had a similar thing happen on a 49. The problem was caused by one of the caster shims slipping out of place by about 3/4".
This shifting was due to it being a replacement caster shims that I bought from a spring shop. The hole in it was too large to be captured by the center bolt which holds the leaf springs together, allowing the caster shim to shift out of place. It occurred during a sharp turn. It was a real head scratcher until I saw 3/4" of shim sticking out.
