The 21 is an indication of the true bore diameter, not the year of production. It means that the gun is slightly underbored. Standard 20ga bore diameter is .615, 21 is .605. If the bore currently measures .615 or greater, the gun would be out of proof under the British rules.
The word choke means that the barrel in question has more than a minimal amount. Surprised that the R barrel isn't marked choke as well, because .009 is enough to warrant the choke mark.
The V mark is for view, from the proof process. It was no longer used after 1904. 20 over C in a diamond came into use in 1887, so I think your gun is from the 1887-1904 period. And it would appear to lack nitro proof, because during that period it would have had either the maximum load data, or else the words nitro proof plus the shot charge and the word maximum. Also, if it were post-1904, the V mark would be absent and you'd see a BV under a crown (not script, easy to read) instead.
I'm guessing that what you're seeing as a BS under a crown is actually a BP, which would be a correct Birmingham proofmark from that period.
Looks to me pretty much standard marks, 1887-1904 period, with no subsequent reproof.
