Keeps a ten-mark note up his nose

The story:

Right idea, wrong currency. I didn't recognize (until I saw it
written out on this site, from somebody's different wrong version
of this line) the slang term for the British pound, so my mind
supplied the Deutschmark. I thought it made the title character
seem that much meaner to be keeping a German banknote up
his nose. Of course I never considered the difficulty he would
have had to obtain a ten-mark note.