Mark Stevens said "... the [Claud Butler] numbering system is split into Year Year /Month and then the number produced that month. The YYM is usually the top but I suspect there has been a misstamp here. ..." and "... There may also be a letter stamp under the bracket which is the builders code...."
My 1953 New Allrounder is stamped '6811 5310' in a horizontal string, so practice was inconsistent as Mark indicates.
Mark, thanks for the tip on the builder's code stamp! That's new information to me ... I will take a peek.
Peter
Peter Brueggeman
La Jolla California USA
4peebee(at)peterbrueggeman.com