It was common for the inside of the gas mask canister to have a simple paper label on the anti-fog lens insert compartment, with the name of the soldier typed or hand written. These small typewritten labels are perhaps the most typical wartime style. For more information about how uniforms and equipment were marked with names, see this article.
Here is an example of a pre-printed label being used for this purpose. You can see that the unit is printed on the label here. Presumably, these unit-specific labels were printed at Bataillon level, for use by every Kompanie.
You can see that the old unit designation of this unit is crossed out and the new one inked in. This designation changed in April 1940. This can was made in 1942, so the obsolete labels remained in use for at least 2 years after the designation changed.
Note that the last line is for “number of the can” which is the unique number assigned by the gas protection NCO to keep track of the unit’s masks. The number of the can is not the ID disk number, the Soldbuch number, the Feldpost number or anything else. Typically this number would be painted on the outside of the can. Here are a few original examples.
I made a reproduction of this printed label with a dotted line for the unit so you can stamp, write or type any unit desired and use it for your own can as part of your living history impression. Here is a link.