Most all 1200's that I've seen have impressed checkering. It can be recut to positive diamonds. I would say it is around 16 to 18 LPI, but that would need to be checked with a checkering gauge. If it is 16, you can tell it with a ruler, and they'll be a line every 1/16th inch. Anyhow, you'll have to cut two starting lines, in both directions, with a single cutter head, then use a guided spacing cutter after, which matches the correct LPI to do it right.
You can make a gauge by tilting a ruler between two lines drawn an inch apart. Tilt the ruler until you get 18 divisions (1-1/8" between the 1" lines), and mark the 18 divisions off with dots, then draw straight lines at each dot to the edge of the paper or card.
