This is from the 1978 Simmons Hdw. catalog when guns were listed by price ie. "Dollar Grade". The price specified the wood grade, checking, engraving patterns, and damascus quality.

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After the introduction of the top lever guns about 1880, the guns were assigned a letter grade.

But it's complicated frown
In the 1882 Parker Bros. catalog the Qualities R, S, T & U were listed with TWIST barrels, and the Qualities N, O, P & Q were listed with FINE ENGLISH TWIST barrels. Ribs were marked TWIST, PLAIN TWIST, or STUB TWIST. Quality P guns have been found with Damascus Twist barrels, but marked Twist and Stub Twist.

[Linked Image from photos.smugmug.com]

Stub Twist

[Linked Image from photos.smugmug.com]

Refinished barrels marked Stub Twist but are Plain Twist

[Linked Image from photos.smugmug.com]

Damacus Twist marked Twist

[Linked Image from photos.smugmug.com]

Your barrels originally looked something like this

[Linked Image from photos.smugmug.com]

BTW I've never seen the 'M' that is on your barrels

[Linked Image from photos.smugmug.com]

In the Hammerless guns the Qualities PH & NH are usually listed with FINE ENGLISH TWIST barrels.

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