Hello Karel,
From memory, you cannot directly address, from it's position in a loop, a device with the standard HP-IL commands of the HP-75C/D.
You must first do an "assignio"and fill up hole in the addresses with fake names.
setup: 1:HP82161A, 2:HP82169A, 3:nothing, 4:nothing, 5:HP7470A
type: assignio ':ca,:ib,:x1,:x2,:pl'
type: printer is ':pl'
type: print "PDPU"
If it's not working, I can configure your setup at home late tonight and make it works.
I will suggest to change the plotter IB address to a lower one so you have less filling to do.
Let me known if the above solution worked.
Sylvain