The H. Gyllenborg that wrote letter 4343 to Sloane is most probably Henning Adolf Gyllenborg, a Swedish statesman and Count.
Reference:
H. Gyllenborg to Hans Sloane, Unknown Date, Sloane MS 4053, f. 136, British Library, London.
Henning Adolf Gyllenborg, Wikipedia, [https://sv.wikipedia.org/wiki/Henning_Adolf_Gyllenborg, accessed 23/08/17]
Dates: to
Occupation: Unknown
Relationship to Sloane: Virtual International Authority File: