There is a reason that catalogue publishers copyright their stamp numbers; those numbers are as much the result of editorial decisions and dare we say it, creativity, as they are the result of mechanical assignment.

Although there is little controversy in assigning numbers 1 and 2 to the first two US stamps, it seems a little perverse of Scott to give numbers 3 and 4 to the official imitations produced decades later. In fact, it is peculiar to give numbers to these objects, which never were never valid to pay postage.

What catalogue numbers really are is a form of shorthand. "1" is shorter and simpler than "1847 5c", and collectors recognize it, just as "C3a" is shorthand for "Inverted Jenny" or "invert error of the 1918 24c airmail stamp".

StampData avoids catalogue numbers by relying on descriptions in those places where it is necessary to refer to a particular type or types.

The lack of catalogue number is both an advantage and a problem. It's an advantage because it gets us out of the problem of deciding numbers, renumbering, etc. It's a problem because collectors have been accustomed to use numbers; dealer stock and price lists are in terms of catalogue numbers.

You can use private tags to notate types with familiar catalogue numbers. Private tags are allowable as fair use, just as if you wrote down numbers on a piece of paper. But making those tags public is not allowed, it violates both copyright and StampData's terms of service, and they must either be made private again, or deleted.