I don't believe that there is such a thing as "P" or "Plenum Rated" sealtight. I'm from Chicago, and we alwways had [and still do], a very stringent code for plenum areas. All we could use was sealtite fro some time, along with plenum rated boxes and covers, and special city of Chicago Plenum Rated fixtures. Not too long ago, a new product called FMT,[flexible metallic tubing], was put into use. It would seem to be a corrugated metal version of what in other parts of the country is called "smurf tube". It's intention was to decrease the amount of PVC [covering the fixture and box whips], in plenum spaces. You can imagine that we have a huge number of high rises, and data processing centers, as well as the stock and futures exchages here in Chicago, so the exposure of a problem was great.
While the cost of things penum rated has dropped shomewhat with the proliferation of things like plenum rated cable and wire ties, I think that if it was made, it would be $$$.