How To Get That Last Stamp You Need For A Set?

Oops, I should have mentioned that US Scott 5-9 are imperf whereas 18-24 are perforated, so, while they do use the same design and are printed from the same plates, the difference between the two groups is obvious at a glance. So in any less-detailed catalogue system, 5-9 will be given one number and 18-24 a different number. So it's not 12 to 1 but 5 to 1 and 7 to one, fly-speckers versus generic collectors.

In other words, the Stanley Gibbons system complicates early Great Britain to about the same degree as Scott complicates early US, about 4-1 or 7-1.

My bad.

In my main specialty, which is Portuguese colonies, I find it more difficult to find some low value stamps that I don't have, than high values. I agree with all that it is a continual search.

There are other catalogs besides the main ones mentioned. These come in handy after one has collected for a while and has decided to specialize in one country, or a set of countries. To mention just a couple, Afinsa in Portugal publishes catalogs for both Portugal and the former colonies. There are catalogs for the French area, like Yvert and Dallay. For early Mexico, prior to 1910, having the Follansbee catalog is almost a must.