According to Peter Labbett’s book the identifying features of the Mk VI bullet are that it is 32mm in length, weighs 215gns, and has one wide cannelure approximately 3mm from the bullet base.
I have one of the magnetic Mk VI rounds as described by Rich, headstamped ‘K12 VII’, and I have pulled the bullet. It is 34.37mm in length and weighs 227.2gns. It is perfectly smooth and does not have a cannelure however there is a very faint mark around the bullet about 6mm above the base which could have been a cannelure - before the bullet was swaged. The lead base is perfectly flat but the core appears to have been squeezed out of the jacket very slightly and is proud of the base by about 0.5mm. I’d say my round has been loaded with a swaged 8mm Mauser bullet. I don’t have a British-made 8mm Mauser bullet to compare this with to confirm whether the faint mark would have been a cannelure or not.
I have also pulled my only British 7.65mm Mauser (made by Nobels Explosive Co in 1917) and found the bullet to be absolutely identical in every respect to a .303" Mk VI. It is 31mm long, weighs 214.2gns, has a non-magnetic cupro-nickel core and has one identical cannelure in exactly the same position as the .303". It also has a very pronounced concave shape to the base.
I am satisfied that my .303" round has been loaded with an 8mm Mauser bullet and this would go along with Roger Mundy’s theory that Kynoch had a surplus of 8mm Mauser bullets & Mk VII cases, put the two together and sold them off to shooting clubs.
The really confusing point now is that Rich has also pulled a couple of his rounds but found that his bullets weigh about 214gns - the correct weight for a Mk VI bullet. I don’t know the first thing about ballistics but I can’t believe that bullets with a 3mm difference in length and a 11gn difference in weight are going to perform reliably.
I’m sorry to have added to the confusion!