This was on the Internet Gun Club website:

Edward Chamberlain was established in 1889 at 1 Bridge Street, Andover.
In about 1907 a branch appears to have been opened at 27a Wote Street, Basingstoke, Hampshire.

Edward appears to have retired or died in about 1910, the name changed both in Andover and Basingstoke to Edwin Chamberlain (son?) in 1911.

From some date after the First World War the firm was named B Chamberlain.

The Basingstoke branch closed in about 1926, the Andover business closed after 1939.

Other Info
The firm sold cartridges under the names "The Smokeless" and "The Universal".

Doesn't say anything about George Gibbs in Bristol but he probably worked for them before going out on his own.

Nice find - probably in 450 BPE - the "magnum" of the day.

John