One of the interesting finds pointed out to me by an ammunition collector at Knob Creek is this interesting tidbit of history. The .5 Vickers is a semi rimmed round that was invented in the last year of World War One by necking down a .600 Nitro Express round. It was intended for the water cooled Vickers heavy machine gun and was initially needed for anti-aircraft gunners to take down German bombers over London that could sustain multiple hits from the .303 cartridge being fired by Vickers anti-aircraft crews at the time. Although the round is the same caliber as the 50 BMG, the two are not interchangeable.
In the 1920s and 30s, there was an attempt by the British military to create a high velocity variant of the round going at over 3,000 feet per second. Bear in mind that the .5 Vickers round was traveling at around 2,650 feet per second while the 50 BMG of the same bullet weight clocked in at around 2,798 feet per second. There were only 40,000 such rounds manufactured in the 1930s, with another 600 or so in the early part of World War Two (probably only for testing and evaluation). These lots were made in increments of 10,000 with tracer and ball versions of the 690 grain weight, and tracer and ball versions of the 664 grain.
The vendor that showed me this find found it for sale at Knob Creek and allowed me to take a number of pictures of the original cardboard case it came in, along with 10 live rounds from the 10,000 that were made. Posted on the right side of the individual round is a modern day 50 BMG round for comparison.