Well John has covered so many hypotheticals very well,I will bring to the table some more likely ones,Old often machined cylinder heads on most cars first thing I would check is that the head has been machined square and all the chambers measure up with close to even combustion chamber volumes.
Plenty of engines have been cooked in the past and someone just machines the head flat and bolts it on with a new gasket,Most bend up in the middle so when refaced end up with more being machined from the ends of the casting so the #1 and #4 cylinder chambers can be a lot higher compression than the centre cylinders.

Especially with an A series engine where pushrods and no cam tunnels like an OHC engine mean no warning sign of a Cam binding when you try turning it often means many now rather elderly A Series heads are best carefully checked or they will detonate on the end cylinders because of those problems.

First easy check for this problem is a straight edge along the Rocker Cover gasket surface,They were originally machined flat and square to the head gasket surface when they left the factory so a simple test like this will show any potential faults before they cause a problem with a built engine.

Mostly either Ring Gap as John pointed out or the above scenario as I have elabotated on are most often the cause of detonation caused Piston Damage or Seizure related stress failure's.