I've numbered them so we can be sure which ones we're talking about

Now redwine and Malcolm mention it, I can see the clear difference in 4 and 6 on the distance to the rim from the tip of the F. I'm still not seeing differences in the serifs though.
However, I do see a new variant in 4 and 6 which on the scan I thought was just a trick of the light but it's clear in hand, under a lens, as well. On all the others, the upper arm of the K disappears "behind" the upright of the F without losing definition - it just carries on till it hits the F. On 4 and 6, the K-bar becomes less distinct a few tenths of a millimetre before it hits the F, giving more of an impression of going behind it.
On the "bronze" one - the underlying metal appears to be yellowish rather than reddish, but I'm not certain whether this indicates plating or just that the alloy oxidises to a redder than normal state because of a slightly different composition.