Whoops. My bad. That's where it should have been. All of Columbia's bikes have the Serial Number stamped near the rear drop outs. Colson had them on the bottom brackets.
The number -- as I suspected -- doesn't appear on most Columbia SN tables. You can contact Columbia's Customer Service folks and ask them if they can peg the production year but I'm not sure if even their records are complete with regard to the tandems.
This is a link to the most common Columbia serial numbers. http://oldroads.com/co_sn.asp
As you can see, no mention of T (which must = tandem since it appears in front of all of the tandem serial numbers) which suggests a different number system was used, e.g., the one I mentioned above is a TS....
If I was a betting man, I'd "guess" the period of manufacture as '63 to '65, assuming the second digit was an "O" and not a "Zero", but that's really a WAG.
Let us know what, if anything, Columbia can share regarding tandem Serial Numbers.