The Makita 763619-3 3/8" collet DOES indees fit the RT0701C Makita router. I couldn’t find any info on this, other than seeing it mentioned in a user manual. I took a chance and ordered it and it fits!
Where did you get the collet?
http://toolmarts.com/ in Cali. I think it was like $13 shipped. Can’t wait to get some big end mills!