Hey folks, thanks for all the suggestions thus far.
So far I think the sand and dowel related suggestions are the most solid as they don't necessarily involve doing anything deleterious so the flask and I intend to use it.
As for the "patina'', I wouldn't call a dent patina
Partially because it is something that technically affects the function of the flask, reducing the overall volume, and partly because patina is more of a film of time and a dent is more a symptom of use (or in some cases abuse). A dent could be charming in some cases, but I think if it were something as in this photo it's more of a detriment. Also, someone has clearly polished all the patina off this flask at some point so there's a lot less to worry about preserving.