Hi!
From reading immumerable Heathkit and other American circuit–diagrams I can confirm the square, triangle, half–moon, triangle, etc., etc., always indicates POSITIVE capacitor terminals with can common negative – I think "Sam's Photofacts" key each symbol to the relevant capacitor on the circuit diagram!
Usually the square is the one used for the reservoir (rectifier output) rated section, whilst the others are for lower ripple–current rated sections for decoupled h.t./l.t. lines!
British multi–section electrolytic capacitors nearly always colour–code the tags, these can be looked up in older Radio and TV reference books, but RED is nearly always the reservoir, YELLOW first smoothing, GREEN second smoothing and so on!
Both British & American twist–lock capacitors often have a hole punched in one of the twist–tabs to allow wiring, etc., to be connected to the can (negative) terminals!
Chris Williams