I came across a post in backpackinglight.com that may (or may not) shed some light on that question:

"It looks like Micropur comes in lots of variations. The one approved in the US is Micropur MP1 which is uses chlorine dioxide like Aqua Mira. The Katadyn site for Canada shows Micropur Classic (based on silver ions, not effective against viruses) and Micropur Forte (silver ions plus chlorine to inactivate viruses). They also have something to neutralize the chlorine taste. I wonder if it would work on the taste of MP1 as well. Unfortunately MEC only sells MP1. I wonder if the others aren't approved for sale in Canada either. I suppose these might show up on ebay occasionally. Do you think it's illegal to ship these tablets to the US if they're not approved? I know that Aqua Mira isn't approved for sale in California, but I think it's still OK to buy it online from an out of state retailer."

Sue