I recently acquired an Allen Pepperbox. The only markings are on the left side of the hammer and read "Allen Thurber, Grafton Mass". It is the 36 caliber Dragoon model with flat main spring and sharply curved handle with trigger and trigger guard. It also has tension screw for the main spring.
It does not have the peg locking system mentioned by Flayderman.

It has a nipple shield nicely fitted that appears to have been silver soldered into place. Is this a later addition, or was it made this way?

Were the barrels rifled or are they smoothbored?

What is the approximate year of manufacture?