You have purchased a Model 48 Columbia Tandem most likely from 1897 or 1898. I have attatched a link to two pages from the 1897 catalog. It is actually quite an amazing tandem and the frame design, construction, and material was a tremendous step forward from earlier designs. It is made from 5% nickel steel which was the highest technology at the time. If you ping on the tubes with your fingernail you will see that the tubing is very thin. Of course yours does not have the original handlebars a few other parts and it is missing the front brake assembly.