Which preposition to use with topis

on Occurrences 1%

There was the engine shrieking in its anxiety to start; there was our luggage neatly spread all over an empty compartment; there was Autolycus protesting shrilly that the train could not leave without his sahib, who was a very burra sahib; and finally there we were with scarlet faces, topis on the backs of our heads, surrounded by a thick cloud of dust, careering wildly into the station.

Which preposition to use with  topis