Here's what it says at the Head Bartender's site:
"Some dedicated Leica nuts are just sure their Leica will be better if it has another finder in it. Upgrading the M2 finder to the .72 M6 finder is easy. I have also heard of some people paying the extra dough to have a M4 finder in the M3. It can be done, but it's a custom job and expensive. Likewise a M3 can be put in a M4 if you are willing to pay the bill, but this makes little sense today with the .85 M6 being easily available. "