the long plate designation refers to a 17mm length plate structure (versus 14mm) inside the glass envelope while ribbed/smooth plate is the 14mm plate length either smooth or with a couple horizontal 'ribs' across the plates.
Sonically, any real Telefunken miniature nine-pin is great although long plate and ribbed plate seem to be in greater demand recently
Sonically, any real Telefunken miniature nine-pin is great although long plate and ribbed plate seem to be in greater demand recently