The most popular filter canteen is the Bartholomae patent.

I made my reproduction based on details of 10 surviving originals canteens.

For sale here and more info here.

Mine does not have the filter because I have not been able to locate an original to study.  The originals were often lost as it was just a small bullet like cartridge that was placed in the slanted spout.   The body of the canteen functioned just as a normal canteen would, the spouts were the difference. The funnel shaped spout was used to fill the canteen not to filter the water going in as has been reported in Echoes of Glory.  The vertical "spout" is not a spout at all but a storage container for the filter when not in use.

There are some canteens that are incorrectly called filter canteens. They did not have a filter but did have multiple spouts.  The Model 1858 French canteen looks similar to the Bartholomae but as the name shows it antedates and possible served as the inspiration for the later.  The second spout is not for a filter but instead ventilation.

This style was imported from France by the Confederacy and used by their troops.  This has been called a confederate patent canteen but the Nuckolls canteen was actually made of wood and had no filtering method.  Most filters were designed to be used with canteens that were already in the possesion of the soldiers as shown in this patent drawing.

This one is housed at the Atlanta History Center.

This one has the blue wool covering.

This one has the two stoppers

There is alot of variation in the Modèle 1858 Canteen.  Ventilation spouts were angled and straight, some were seamed on the front others on the back, and some had wire keepers while others had straps of tinplate. Some of these were made by Alexis Godillot in Paris, a famous manufacturer of military supplies.

This is the original I copied.

For sale here.

The Modèle 1858 French canteen was still used during World War One as there was a surplus even thought a new version was introduced in 1877.

It was replaced with this model which continued to have the ventilation spout.