18:51 Farthing real variety or start of a simple cracked die.

[absence of uniformity]

If you look at the bottom image you can see the faint line of the die crack. The coin was listed/sold as having a colon.

I'm aware a variety is listed for the 1851 Farthing with a purported colon.

Looks like the beginnings of a die crack rather than a colon to me.

 

[absence of uniformity]

[Coinery]

I agree, almost certainly connected with the die crack!

[Paddy]

Could be they drilled a small hole in the die to limit the crack. It is a well known technique to drill a small hole at the end of a crack as it reduces the stress at the tip and so makes it less likely the crack will propagate further.

[absence of uniformity]

1 hour ago, Coinery said:

I agree, almost certainly connected with the die crack!

Both coins are missing the serifs on the B of BRITANNIAR.

[absence of uniformity]

56 minutes ago, Paddy said:

Could be they drilled a small hole in the die to limit the crack. It is a well known technique to drill a small hole at the end of a crack as it reduces the stress at the tip and so makes it less likely the crack will propagate further.

Interesting, do you think the images show that progression? A crack formed, two holes drilled and then the die deteriorated further to show the pronounced crack we see in the first image.