Hi John,
I need to correct my earlier post regarding the "OIL HERE" stamping. Gordon Bruce does in fact mention on page 30 that this instruction was first stamped on Mk1s wit s/nos between 45500 and 46500 so my fairly late 48316 conforms with this.
On the other hand Bruce is fairly clear that the shorter barrel was not introduced until the time of the modified shape with the 120 degree slope so maybe you are right and the shorter barrel on mine was fitted later. Who knows?
Without re-reading these chapters again is it clear when the smooth bore barrel was introduced?
Aubrey