Pins 3 and 9 are both control grid (g1) terminals — dual grid connections for suppression of HF parasitic oscillations, common in TV output pentodes.
PL82 is electrically identical with a series heater (45V/0.1A) instead of parallel (6.3V/0.8A); same electrode characteristics but not a pin-compatible substitute.
D=10% is the standard European rating condition used to specify output power; not a typical audio operating point.
Triode-strapped data sourced from Tom Schlangen, 'Overview of pentodes / BPTs strapped as triodes' (September 2009), not from the manufacturer datasheet. Measured plate curves available at http://www.tubes.mynetcologne.de/roehren/daten/el82pentode_as_triode.pdf
Pin Layout — Noval
1No Connection (internal screen)
2Suppressor Grid (internally connected to cathode)