Early War office twins were produced in 1933. The J34-11, J35-11 en J12 for civilian production were produced in 1934-1936.Prefixes to the engine and frame numbers were B11, E12, D12 respectively.
Interestingly enough is that 1936 J12 models were despatched for ´New Scotland Yard` as it says in the despatch records in December 1937 up to the beginning of 1938. These had diecast crankcases, air hardened cylinders and a `full blue´ petrol tank.
The War Office twin was also made in those years but the RAF ordered more and these were produced in 1937 with the H prefix to the frame number.
War Office twin differed in specs, e.g. no tank instrument panel but headlamp panel and some other small changes. They also had other prefixes: B15, E15 and D15 respectively.
Since I am currently at work I don´t have the book ready but I am pretty sure this accurate.
I will check for the oil pipe but I am petty sure that what Mick says also goes for the other years.
I also have a J11. You wouldn't happen to have a few spares lying around like rocker boxes? I know they are like rocking horse manure but just asking anyway.