Hello,
Can anyone tell me what the engraving means, and wherefore it is used?
22 mm
1839 Victoria, Farthing??
First: yes, it's a farthing.
On the SB: unless this has been documented as a known trader's counterstamp, it's unlikely you'll ever know for sure. It could be a love token given by SB to his/her sweetheart; a tool check (though these tend to have a hole); a trader's makeshift token or tally...
It could have been modified for any of these purposes after the big copper coins ceased to be legal tender. The bronze replacements were brought in in 1860; I don't know precisely when the copper ones were withdrawn, but presumably soon after that. The wear on the coin is consistent with a good 20 years' honest use.
Alternatively, it could have been modified by someone overseas who had no other use for a foreign coin. French coppers countermarked by traders for use as tokens in the UK are quite common, as are Swedish ones used in the same way by Danish businesses. In the case of the UK it was illegal to deface coin of the realm, but you could do what you liked to other countries' coins. I don't know though whether the prohibition applied only to current circulating coinage or British coins in general, regardless of legal tender status.