I am trying to identify this ‘relic’ on behalf of a metal-detector enthusiast who found it on farm land here in Norfolk, England. I know absolutely nothing about these larger machine gun rounds and, from my army experience, I initially thought .50" M48 Spotting Rifle but having cleaned the base it clearly is not. The headstamp is ‘4 3 T W’ so it’s obviously a wartime item. He regularly finds .50 Browning rounds and cases in the same area and there was a lot of USAF activity around here during WW2 including an aerial gunnery school a couple of miles away.
The caselength is about 70mm and I’ve pictured it alongside one of his .50" Browning rounds for comparison - and as soon as I compared the two I noticed the big difference in extractor groove width. Also intriguing are the two tiny holes in the neck - are they simply the result of corrosion or are they intended to be there?
What is it?