You found left/right markings ? Good catch !

re: polarity, isn't the speaker signal going to be AC, no significant DC component ? My assumption had been that direction wouldn't matter in that case but maybe I missed something...

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8