[Jac]

Thanks John,

That makes it even more complicated.
The 383A/663A has a mains transformer and voltage doubling with 2x HVR2 from the anode of the line output valve.
To me it is not clear how the line output transformer itself is built, because it is in a sealed 'box'. See attached picture.
(I have no X-ray equipment to look into it.)
It might very well be a laminated iron cored transformer with several wave wound coils, as was not unusual at the time.
Perhaps there is petrolium jelly in the box, as is in the transformer of an experimental Philips projection receiver of an earlier date.

The 385U (no mains transformer - 2x EY51) is dated at October 1949 according to the Trader Sheet (nr 993).
The 1100 ditto August 1951 (Trader Sheet 1051).
The 383A/663A certainly is before the 385U.
Unfortunately I could find no Trader Sheet for the 520A of Mullard equivalent.
There must be date codes in it it somewhere however. Perhaps on the CRT as well.
The Wireless and Electrical Trader has an advertisement of the 383A at January 8, 1949.

The 520A has, as David pointed out, a Birmingham version.
The 383A/663A has not.

So the sequence looks like this:
* 463A + 563A (date??)
* 383A + 663A (early 1949?)
* 520A (1949?)
* 385U (Oct 1949)
* 1100 (Aug 1951)

Jac.