31 model 20 piston ?

I hope you manage to get to why this occurred ,  as  a repeat will be costly  and sometimes dangerous to your health.  How much clearance was this piston fitted with?. Is the carburation weak? Ignition timing correct. Oil getting around ?. Bore true?.

My suspicion is that the carburation is wrong . New does not always mean right. I have recently had to bore my own needle jet  as the  new one  just would not work. It may be that the new carburation was wrong from the start but usage /weather / fuel  has  brought this on. Clearance and timing sound OK. Check for swarf in the jet block. ,Restricted fuel supply ( my tank outlet and fuel tap bore was  almost blocked solid with  tank liner. and that was before ethanol got to it !!)

If the Carb has been set up by a distributor  did they have the 1930's  notes on   any special  machining requirements  or perhaps a  1931  Norton in their  workshops  to  check how it runs ? . Perhaps not !!.   I ran into this   issue  when  building up a  pre-monoblock carb for  a Rudge Ulster. I learnt  that the  normally found  machining for a 500 cc carb was not  any use  as  back in the 30's Amal  bored out the  bypass and  pilot ways  and individually flow checked carbs for Rudges along with chrome plating the bodies and making special flange spigots , stepped cable entry, etc. I would like to see a copy of the 1930's Amal carb specs  , Could be interesting.

That was a serious seizure  ,looked like  a four corner  one.  Usually  from a   much overheated engine or one with too little clearance.A Rudge from the same period would also often seize with less than  7 thou  on a Gold Star piston if worked hard. Rudge cylinders are reputed to go out of shape when hot.Perhaps your cylinder needs a carefull measure up.