Mullard Tube Code Help

Hey everyone,
I just picked up a pair of Mullard EF86 tubes for cheap and I am trying to date them. The codes on the tubes are:

9r2 B2O2
9r2 B2D5

From what I've been reading, the 9r2 bit refers to the tube type and change code. The B refers to being manufactured at the the Blackburn plant, and the 2 is the last digit of the year.

And I thought that the O and D were supposed to refer to the month of manufacture and the 2 and 5 to the weeks, but if so then what month does O mean?? I'm guessing I have it wrong, or I'm using a tube code pattern from earlier or later tubes.

Thanks for the help!