Sorry, I have way too many Bug Reports saves cluttering my saves folder -- turns out I opened a different poster's Catherine save when I said I loaded the turn 120 save, so apologies for that confusion.
OK, I've opened your turn 761 save and here's what I see:
You do have 5 coal (per the top of the screen), but still only have 4 coal in your cities and no coal from CSs over which you are suzerain, so you must be getting coal from a trade deal with another civ -- that's the only other possible source of coal in the game.
Unfortunately, the game does not include a full deal history screen (hopefully that will come in a patch), so I can't confirm which civ might be sending you coal, but when I look at your gold income from trades is see that at turn 754 you had 5 trades with Brazil, generating +24, +10, +48, +39 and +3 gold. Seven turns later (turn 761), you still have 5 trades with Brazil, but the order and gold yield of the trades has changed: +48, +39, +3, +38 and +34. This indicates to me that the first two Brazilian trades in the turn 754 listing came up for renewal during those 7 turns. I think Brazil is the most likely potential trading partner, because on turn 754 he has 2 coal available to trade, but on turn 761 he only has 1 coal left.