It is a high condition gun. I believe with the straight stock, checkering, border engraving and Twist barrels it would be a $75 grade.
1874 Price List
This is another Plain Twist lifter in much lower condition
https://www.gunsinternational.com/gu...n_id=102415781
Value is significantly related to whether the gun can be used, which requires answers to these questions:
1. What are the chamber lengths?
2. Does the gun have step chambers as Dean suggested, or short forcing cones?
3. What do the bores measure?
4. What are the end of chamber, 9" and minimum wall thicknesses (and location thereof)
5. Does the gun have chokes and if so what are the constrictions?
Please check with your gunsmith and we could be of much more help with that information.
BTW: what shell pressure did the gunsmith say would be appropriate?