Don't know and I suspect your guess is as good as mine. Out of curiosity, what does the schematic show? Does it show a loop or just an aerial sign?
I have lots of tube radios with built-in antennas and lots without. Shortwave models usually have connections for external antennas and medium wave 520-1610kHz broadcast band radios usually have loops. But TransOceanics have both, a telescoping antenna for SW and a loop for MW broadcast band as well as external connectors. And I have some AM-only (520-1610) that have only an external connector for an antenna, without a loop. Best antenna for those is a few loops around a shoebox.