Okay, with the help of a support ticket, I have got hold of the good version for 4.5.5 It just requires some insider knowledge on navigating the Zimbra support portal ... and a ZCS: Network Edition support account.
1. Go to the Support Portal/Downloads page:
Zimbra Support Portal - Home
login with your support username and password
2. Click on the Search Repository box
3. Enter 'migration' in the search field then hit 'Go' - that will get you a list of migration wizards currently available.
4. You should find and download the Exchange Migration Wizard with a File Version of 4.5.5_GA_881