Looking at the PDF, it seems the part you need is R190,534.
It's funny, because I have the same tripod, and the screw to tighten the telescoping central column became de-threaded last year. I've been using a bolt (and therefore need a wrench to adjust the column) since then, but maybe this'll let motivate me to actually replace the part! If you find out how to actually order it through Manfrotto (or Bogen), let me know, otherwise I'll try the local camera shop and see if they can order it.
Jspitfire From Canada, joined Feb 2005, 308 posts, RR: 2
Reply 3, posted (5 years 4 weeks 1 day 13 hours ago) and read 6700 times:
Thanks Dana, this is looking a lot easier than I thought it would be! With my serial number it's part #R190-504, so I just went to B&H Photo's website and typed in the part number, and there you go, $5 each!