Robert, I think that I may have found a solution. I also have a Hitachi M12VC router attached to my machine, and was also wondering about different collet sizes. SO when I was at a certain hardware store that sold both Hitachi and Bosch routers, I noticed that they also were selling replacement Bosch collets. So I took one of the Bosch collet/nut assemblies, and threaded it onto the Hitachi router that they had on display, and lo and behold, it threaded on. (The Hitachi router that they had on display was not the M12VC type - it was a plunge type, but I am thinking that the collets are likely the same.)
So, I just did a saerch on Bosch collets, and found a link on their website to buy the 3/8" size:
http://www.boschtools.com/tools/tool...60&I=58160&T=1
So I am planning on looking further onto this. I am posting this reply many months after the previous post - but if any of you guys are still interested in this and beat me to the punch on investigating/buying a Bosch collet for the HItachi. please post your results.
THanks,
Mario