12AU7 and 12AX7 long black plates vs short black plates. Any difference ?

Don’t know if any of this info will help you.

FWIW I used Joe’s Tube Lore when buying early to mid 1960s NOS and used vintage early to mid 1960s, 6922 / 7308 / E88CC / E188CC small signal tubes. Joe’s listening evaluations were spot on, IMO. YMMV.

https://www.audioasylum.com/audio/faq/joes-tubes.html#12AX7

I agree that Joe's Tube Lore is loaded with good information. I bought a lot of Sylvania tubes from the 50's and 60's, mostly 5751's and mostly gold brand gold pins. I prefer 3 micas vs 2 and longer plates do seem to be better as well, especially in Amperex BB. 

Thank you. I'll take a look. I currently have long matte black plates Sylvania 12AU7. Seems impossible to find them truly NOS, especially matched.