[Malcolm G]

Copied from elsewhere;
Shell-Mex and BP Limited (SMBP) was formed in 1932 when the Anglo-Persian Oil Company (APOC, now BP) and Royal Dutch-Shell decided to merge their United Kingdom marketing operations. APOC held 40 per cent and Shell 60 per cent ownership, reflecting their respective market shares. SMBP acquired the Power Petroleum Company and the Dominion Motor Company in 1934, and the National Benzole Company in 1957. Each brand continued to be marketed separately throughout the existence of the joint company. The parent organisations de-merged their United Kingdom marketing operations in 1976.

Or from another source;
Shell-Mex and BP Ltd was a British joint marketing venture between Shell and British Petroleum, whose headquarter were located on London's Strand. It was formed in 1931, partly in response to the difficult economic conditions of the times, and operated in the British Isles only. It was eventually disbanded in 1972.