perhaps a Mk1/2 pillar/A panel was grafted in during the external hinge conversion?
Admittedly without knowing how much work was done here, I'd suggest it looks more like no reinforcing at all was added during the conversion to external hinges. Moving the weight of the (far heavier than Mk1 type) Mk3 doors onto the A panel without beefing up the post will cause this kind of crack which as shaun617 has pointed out, in this case appears to have some age to it.