I agree with SensibleSal, this is most likely to be a Chinese chop mark. My Spinks, when referring to the 1887 Jubilee gold £5 and £2 coins, says “Beware of recent forgeries”. These 2 coins were produced in limited quantities and only in 1887, really as souvenirs of the Jubilee rather than as coinage for general use, and so if a Chinese merchant in – say - Hong Kong, if he ever saw one of these, would probably scratch his head and wonder if it was genuine and what was the face value. If so, once he had accepted one of these coins he would no doubt authenticate it with his chop mark.