In fact they are UK. This is what IGC lists for both, though my suspicion is that they are one and the same firm:

Name Alan & Thumber
City/Town Worcester
County Worcestershire
Country United Kingdom
Trade Gun & Pistol Maker
Notes
6 shot pepperbox revolver sold.

&

Name Allen & Thurber
City/Town Worcester
County Worcestershire
Country United Kingdom
Trade Gunmakers
Other Address
Dates 1840
Notes
Allen could have patented a pepperbox revolver mechanism in 1837.

Tim