George this could be a sticky post so we can add info on a day to day basis....
Larry feel free to add your grain of salt here, anyone with accurate info please let us know here, pics are welcome also........
ok here we go................
Mullard: The Long Plate Mullard (lenght about 7/8") and the Mullard with the box anodes (known as CV4004 and M8137) aren't really good sounding in guitar amps. They especially were made for HiFi use and as Koa-dogg already mentioned, they are more even in response.
The "Real" Mullards are the I61 type, made from 1959-1962 and the I63 type, made from 1963-1969
The I61 is bearing the Mullard shield logo, below is written from top to bottom: ECC83 - BVA in an elliptical circle - BRITISH MADE
The I63 is bearing the Mullard shield logo, below is written from top to bottom: ECC83 - 12AX7 - BVA in an elliptical circle - MADE IN GREAT BRITAIN
On all these Mullards you'll find an etched code in 2 rows near the bottom of the glass envelope.
The first row has 3 digits: I61 or I63
The second row has 3 or 4 digits, but always is beginning with the letter "B" (for Blackburn in UK, where these tubes once were made).
