The original JP5s were mostly converted to JP5A in around 1974-5. A few JP5s were allocated to Finningley for navigator training; these were fitted with tip tanks, the moustache strakes were removed as was the roughened material on the wing leading edges. These were the only JP5s left in service and were never officially designated JP5B.
Spotters persisted in referring to them as JP5B, just as they referred to the F-4J(UK) as the 'Phantom F3' - but neither were officially used.
The fin spar cracks were, if memory served, tracked down as being caused by icing on the fin-mounted VOR antennae causing antenna flutter and stress to the fin. One pilot saw this when looking in the mirror once! One aircraft was nearly lost, but the solution was simple enough - bracing wires limiting antenna movement.