When I responded on this bike on the CABE, I missed that you had posted the serial number…
CWC serial numbers ran to A99999 and then restarted with B00001. The inclusion of bicycles with both “X” and “Z” serial numbers produced in late 1936 and early 1937 confuse matters, but I hold the break from 1936 to 1937 at falling near the switch from “A” to “B” serial numbers. That would put your bike into early 1937.
The frame type of this bike is what I refer to as the “Bent Tank” for the distinctive shape of the tank aperture. Obviously I also refer to the tank that fits this frame as the Bent-Tank!
The BT frame was produced from mid 1936 until the outset of WW2 with a number of small construction variations that define the age of the frame along with the serial number.
The matching tank was also modified several times over the course of its production.
Here is a link to the original patent drawings for the tank:
http://www.google.com/patents?id=Hm5kAA ... le&f=false
There are two possible variations of the tank that could be correct for this bike. Both will have a removable panel on the drive side that has a battery cage welded to the inside of the panel. The panel screws independently to the tank and sits in a grooved trough that surrounds it. A similar grove is stamped into the offside of the tank to visually balance it.
The earliest versions of this tank have no louvers and no horn button as the earliest bikes were fitted with an external pancake horn. The second version has a horn button located mid-side on the offside of the tank to operate an internal horn. This version also has louvers to allow the horn’s sound to escape. This same basic tank was used in 1938 but the horn button was moved to the top front corner of the tank. Later versions of the tank no longer have the removable door, but retain the stamped groove and the last versions have plain, unstamped sides.
These tanks are not easy to find but they exist in numbers large enough that they do turn up for sale. You could also correctly build the bike without a tank because a larger percentage of these bikes left the factory unequipped compared to those equipped with a tank.