Petromax 821...How to date??

i want to learn how to date my P'max 821..Help me ..

 

Depends much on the era of the lantern.
Are there some Numbers stamped ore scratched on the bottom?
It will be helpful, if you show some pictures of all views of the lamp.

 

... ah, and also you can do some comparison with shown lanterns in the Reference Galleries & Libraries:
Lanterns

 

I would suggest: start with posting pictures (detals) of your lantern here.
Date stamp on the bottom, scratches?

 

... and some of the complete lantern, please.

 

here u go....

 

My guess : 1930's

 

Appears to be prewar for sure. I think the fiber knob pretty much says that!