What is a Remington 1866 Derringer 6394 worth?

Answer 1

Depends on (real) type and condition, condition, and condition.

I assume you mean a double derringer (model 95)?

Where did 1866 come from?

A type I model 1 couldn't have a serial that high. (and that's the only one made in 1866)

A type I model 2 doesn't have a serial no but a batch no

A type II model 3 may or may not have a serial based on which variation it is but that's starting in 1888.