Rich, that could be a factory relieved block.
My understanding of the reason behind it was to shorten the heat flow path from the cylinder to the ex valve.
I heard the vacuum brakes in trucks leaned off the fuel mixture, causing increased heat which in turn increased the incidence of cracking from the cylinder to the exhaust valve seat.( the block then being of no further use)
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Bluebell
Carrier Armoured O.P. No1 Mk3 W. T84991
Carrier Bren No2.Mk.I. NewZealand Railways. NZR.6.
Dodge WC55. 37mm Gun Motor Carriage M6
Jeep Mb #135668
So many questions....
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