There seem to have been a succession of different models sold under the Colt 5 and Colt 8 names. V belts are normally bought under a standard industrial classification consisting of a letter that gives the belt section width, and a number that is the belt length in inches. (For example, some late model Colt 8 cutter belts were B59.) If you have the old belt, take it to an industrial supply shop and they will measure its length on their simple sliding gauge, and identify the section by just glancing at it, then they'll wander into the stock room and come out with the replacement belt. The new belt will have the letter and number painted on it, on the outside surface, so you can write it down. I take it you've looked carefully at the outside of the old belt, to see if it is still visible?
Joe, V belts don't stretch - they have built-in cords near the outside cover, and the cords are unstretchable, though they can be broken. However as the belt wears it sits down further in the pulley, and this may confuse the length gauge slightly, depending whether it just uses the inside surface or is V-shaped.
