I just purchased some #6 Bock Conklin nibs (M) from the original modern Conklin pens in eBay. They are all smooth and pleasant, and reasonable at $8.00. Depending in pen and feed and ink, they seem to range from F/M to M. Thought someone might be interested in these. The seller was quick, and I had no issues.
http://www.ebay.com/itm/CONKLIN-amp-.../172590037371?