Information on mechanical calculators can be found in many places, but I have found the following sites to be very useful:

John Wolff’s Web Museum – absolutely vital when your Odhner doesn’t work

Odhner Serial Numbers

Odhner Calculator Family Tree

Rechnerlexicon – full of useful information, and not just Odhners!

Rechenmaschinen-Illustrated – Good pictures of many rare types

Vintage Calculators Web Museum

History of Facit Calculators

Facit Serial Numbers

Bell Punch Comptometers

Felt and Tarrant Comptometers