Cores are definitely different. Crown shape for one, and on the reverse, the position of the leaves in relation to the frame.
But I am afraid I don´t see anything resembling "O" at that ring. Maybe it resembles some letter with loop (like B, P, R) but surely not O. In my opinion, of course.
Let's do some hugely rough math. The dates are not accurate, and the numbers approximate.
1914 EKII's awarded = 5,000,000
Time period (1914-1924) = 3625 (assuming start of WWI and finish in mid-1924)
That's a rate of 1379 EKII's produced every day of this period - that were actually awarded, so not including how many more were produced and not handed out.
If we further assume a maker count of 200, that's almost 7 EKII's per day per maker. Assuming further, a 9 hour workday, that's an EKII every 77 minutes (or thereabouts). For 10 years.
All numbers for fun of course Initially I'm sure the EKII production was slower, but once they started being awarded en masse, it would have ramped up.