60's NOS RFT ECC83 (12AX7) - Marking Identification

Found two handy reference sites for the 12AX7 and applications. This link is the most comprehensive:

1. http://www.amplifiedparts.com/tech-corner?art_id=262
2. You can now select "Matched Sections" for most common audio pre-amp tubes; there is an added $5.00 cost per tube that will be reflected in your shopping basket. We've been offering this service for years but have now made it easy to order on the website. This is the most accurate service available to insure that all your pre-amp tubes have equal gain.
   Click the questions below to jump to the answers.
   
   1. What does "Matched Sections" mean anyway?
   2. What do you get when you order "Matched Sections" tubes?
   3. Who should use "Matched Section" pre-amp tubes?
   4. What is the difference between a "Matched Sections" tube and a regular tube?
   5. How do you order matched pairs or quartets?
   
   1. What does "Matched Sections" mean anyway?
   Many of the most commonly used pre-amp tubes (e.g. 12AX7, 12AU7, 12AT7, 6922) actually have two identical sections inside of them. These two sections are technically called triodes and are intended to have equal electrical performance characteristics. It is two tubes in one enclosure. When you pay extra for the "Matched Sections" tubes you are getting the assurance that every triode section in every tube will be as closely matched as possible, ensuring balance.
   
   2. What do you get when you order "Matched Sections" tubes?
   Tubes are manufactured to be within certain tolerance ranges according to a specification but like all electrical devices there are variances from tube to tube and section to section. By using "Matched Sections" tubes the two triode sections inside every pre-amp tube will be identical to each other and also, each triode section will also match every other triode section in other tubes of the same type. You are guaranteed that every triode section of every tube will match the spec for its type.
   
   It is possible to get "matched" pre-amp tubes that are not balanced, meaning two tubes may match each other but their own halves are not identical. Our "Matched Sections" service is superior to this and insures that all pre-amp tubes (of a type) that you have ordered will be perfectly interchangeable with each other.
   
   3. Who should use "Matched Sections" pre-amp tubes?
   For guitar players: if your amp has a "phase splitter" (or "phase inverter") it is important to have matched triode sections in this tube location because of how the tube is used in the circuit. For example, Fender amps often have a 12AT7 that is used this way. For other parts of the pre-amp stage in guitar amps this service is not necessary.
   
   For audiophiles: For the best pre-amp performance possible, use matched and balanced pre-amp tubes.
   
   4. What is the difference between a "Matched Sections" tube and a regular tube?
   Unless the tube is defective it will be within tolerance and will work fine in most applications. Some brands are better than others. When you pay extra for the "Matched Sections" tubes you are getting the assurance that every triode section in every tube will be as closely matched as possible.
   
   Every matched tube is labeled with the triode section measurement. If you just want a regular tube, choose "None". You will get a regular tube that has had no special selection done and the tube will not be labeled.