caddy: No. 00, 0, Field & Ideal grade guns were finished with shellac.
Zinsser Bulls-Eye 3 Pound Cut Amber Shellac is similar to the original product.

Higher grade guns were oil finished. Hunter Arms Co. used "Rose Oil" finish; a formula which included Spar Varnish, Japan Drier, boiled linseed oil, and other secret ingredients.
A brief description of the process may be found in "L.C. Smith: The Legend Lives"

There is more information, and images of original finishes in the "Maintenance and Restoration" section of the LCSCA FAQs
https://lcsca.clubexpress.com/content.aspx?page_id=274&club_id=43784

A 1943 20g Field about as close to factory original as one can find

[Linked Image from photos.smugmug.com]

The action, lockplates, and top lever (and hammers on hammer guns) were case color hardened.

Forend iron was case colored on higher grade and ejector guns; extractor 0, 00, Ideal and Field were Nitre Blued.

Lockplate connector screws may be case colored or blued. FW lockplate retaining screws were case colored. The trigger plate retaining screw was mostly case colored but also blued.

The trigger guard and screws were Nitre Blued, as was the top-lever screw in the trigger plate.

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