Which "pilotless aircraft" did the authors of the 1944 Chicago convention have in mind?
Note that the article says that 


  No aircraft capable of being flown without a pilot


At that point of time there were a number of radio controlled aircraft which could be flown without a pilot on board (for example, the RP series aircraft and OQ-2 Target drone, of which thousands were made). The authors of convention most probably had these in mind when they frame the article 8.

This ICAO document on Unmanned Aircraft Systems also points in the same direction:


  4.3  Article  8  details  conditions  for  operating  a  “pilotless”  aircraft  over  the  territory  of  a  contracting  State.  To  understand  he  implications  of  this  Article  and  its  inclusion  from  the  Paris  convention  of  1919  (Article  15)  into  the  Chicago  Convention  of  1944,  the  intent  of  the  drafters  must  be  considered.  Remote-control  and  uncontrolled  aircraft  were in existence at the time, operated by both civil and military entities. “[A]ircraft flown without a pilot” therefore refers to  the  situation  where  there  is  no  pilot  on  board  the  aircraft.  As  a  consequence,  any  RPA  is  a  “pilotless”  aircraft,  consistent with the intent of the drafters of Article 8. 


(emphasis mine). So, the authors of the 1944 Chicago convention most probably had radio controlled aircraft in mind.