For those of you who followed the thread on the Broad Arrow marks a few
months ago, I found the following passage of interest, and thought you
might too.  This appears to be the introducction point of the Broad Arrow
mark.
 ca

Excerpt from "The Standard Directory of Proof Marks, by Gerhard
Wirnsberger, Published by Jolex, Inc.  ISBN: 0-89149-006-X

Page 101

In 1868 a new proof law was enacted, this one known as the "gun proof Act
of 1868".  Although little new was added, this law further strengthened the
proof rules and their enforcement.  Actually, this was the last of the
British gun proof laws, and whatever additions have been made since then
have been by ministerial additions to keep the law up to date.

At that time the British government was selling huge stockpiles of old
military smoothbore flintlock and percussion rifles.  All of these arms
bore military acceptance stamps, both of the army and the East Indies
Company, but not the official British proof marks since, as military arms,
they were exempt from standard proof.  However, if these guns were to be
brought on the market and were in serviceable condition, many potential
buyers believed that the arms would not require proof or re-proof.  To
resolve this dilemma and indicate that arms were not only legally put into
trade but serviceable, the (broad arrow illustration) Broad Arrow mark was
introduced.  Since many of the barrels were in questionable condition as
well as being obsolete, two additional marks were introduced.

The capital letter S indicated that the barrel was serviceable, while the
letter O meant that the barrel was out of proof and required re-proof.
Either of these letters was applied above or directly on the Broad Arrow
mark.  If the barrel lacked either the "O" or the "S" stamp, it was to be
considered as un-proofed, and therefore had to be proofed.