I come across a decent amount of no name brit boxlocks and have no clue what the right price for them is.

For example:
12 ga
28"
2.75 chambers
Manufacturer unknown or was simply a name stamped on someone else's product.
Stock work done (pad, spacer, etc.)
Ejectors
Probably made in the 50's or 60's

With no brand recognition at all, what would you value a british boxlock like this in decent shape (80% colors)?