Hi Pete,
thank you for explaining 
Meanwhile I’ve found something at the page of our dear Russian Strida brothers
http://club.strida.ru/strida-1986/
It seems that this kind of pulley was created also of at least two, more likely even three, single parts (I think). First a flat stamped steel flange with six holes for the bolts, vice-versa the outer flange/brake drum bowl and between these - maybe clamped - the molded Aluminium pulley part itself.
Would be interesting which way the brake drum and the pulley are connected - that can’t be seen at the pics?
Did you look inside of the brake drum, are there also rivets visible?
Best regards,
Chris