Mullard ECC35 & ECC33

OK, let's talk about these highly sought-after tubes due to their pleasant sound signature. I have searched for quite a while but dont really find any much details on these tubes on the web. I am a super curiosity type of person, and I know someone will say curiosity kills but very fortunately, I am still alive after all these years... What I am really curious about are the followings:

1. Why are there different varieties of black and brown base, tall and short glass? Is it becos due to the requirements for certain usage?
2. Are there tubes that really made in other than Blackburn after Philips acquired Mullard? I have only seen early code 1347 (ECC35) / 1324 (ECC33), later code RU* B** / RU* B***(ECC35) and 5K* B** / 5K* B*** (ECC33), which are all made in Blackburn, UK. The only one that I have read that is not made in Blackburn, is made in Mitcham with code 5K2 R7J3.
Ref:
B : - Mullard, Blackburn
D: - Valvo, Hamburg (Germany)
G : - Mullard, Fleetwood
J : - Mullard (British Tungsram), Tottenham
R : - Mullard, Mitcham
X : - Philips, Sittard (Holland)
Y : - Philips, Sittard (Holland)
9: - Philips, Heerlen, (Holland)

Lastly, I have 3 pieces of ECC35 slim black base and grey plates with no etched code. They have the regular Mullard emblem, BVA logo, BRITISH MADE and DA at the bottom. What are these?
I hope someone with a strong knowledge of these tubes would kindly enlighten me. Thank you and take care.


This is my collection on ECC35 & ECC33 (all are made in Blackburn except the unknown 3 pieces)

 

A fan of both but the ECC32 is my favorite! Love the shape, too.

 

OK, just wanted to share my findings so far.

1. Fat base, ECC35/33 or black/brown base, started from RU3/5K3, that tells me they are of later version compared to slim base. It seems that they started to appear around 1957.
2. Mullard changed its designation code on ECC35/33 from 1347/1324 to RU*/5K*, somewhere in 1955
3. Date code interpretation - 2 or 3 character code (pre 1960) / 4 character code (post 1960)

Lastly, there are differences on the different versions of the ECC35/33 on the physical structure and etched code:

1st version - black plate, pan getter (e.g. code 1347/1324 MM/MN/MO/MS, ~1949)
2nd version - black plate, pan getter (e.g. code 1347/1324 J8/K9/A0 - Oct 1948/Nov 1949/Jan 1950)
3rd version - black plate, D getter (e.g. code 1347/1324 F1/G1/I1 - Jun 1951/Jul 1951/Sep 1951)
4th version - grey plate, D getter (e.g. code 1347/1324 E2/B3/E4 - May 1952/Feb 1953/May 1954???)
5th version - grey plate, D getter (e.g. code 1347/1324 1H2/1L2/1A3 - 1st week Aug 1952/1st Dec 1952/1st week Jan 1953???)

6th version - grey plate, D getter (e.g. code RU*/5K* - * is the change code from 1-4, below are the date codes I have recorded :-
B5H - Blackburn Aug 1955
B6B - Blackburn Feb 1956
B7D - Blackburn Apr 1957 - Fat base (RU3/5K3) started to appear around this period
B8K - Blackburn Nov 1958
B3F1 - Blackburn Jun 1963 1st week
B6F4 - Blackburn Jun 1966 4th week
B6H3 - Blackburn Aug 1966 3rd week
R7J3 - Mitcham Oct 1967 3rd week
B7J4 - Blackburn Oct 1967 4th week

***Note: Date code for 4th and 5th version have really confused me. They seems to overlap unless there is a misinterpretation of understanding the date format. Please feel free to correct me if there are any errors as well as any inputs or information are greatly welcome and appreciated.​

*** Disclaimer: All information is collected from various sources found on the web.

 

UPDATE:
This apply to ECC33 fat base (black or brown) only. To add to the confusion, Mullard Blackburn seems to have made fat base (black or brown) ECC33 with code fX1 B7J/B7K/B8D (note fX1 has a lower case f and an upper case X) around late 1957 to early 1958 for unknown reason. Due to the lack of information, I have no way to confirm this or provide any further details. Please share any info if you do. Thank you.