It's the "new improved" version of the 209P:
http://support.remington.com/SHOTSHELL_COMPONENTS/What_is_the_difference_between_the_209_Premier_primer_and_the_209_STS_primer_for_shot_shells%3F
https://support.remington.com/SHOTSHELL_COMPONENTS/What_is_the_difference_between_the_209_Premier_primer_and_the_209_STS_primer_for_shot_shells%3F
The new Remington 209 STS is a more ballistically consistent model of the present Remington 209 Premier ("P")
The new Remington 209 STS is a more ballistically consistent model of the present Remington 209 Premier ("P") primer. Besides the visually smaller flash hole in the 209 STS primer, improved manufacturing technology has been incorporated into the production of the Remington 209 STS primer. These changes provide more consistent velocity performance in many target load recipes and allow the Remington STS Primer to be used with a wider variety of powders.
Since the new 209 STS uses essentially the same ingredients, there should be little or no change to presently available reloading data, however, you should check with the manufacturer of your favorite powder to be sure your favorite "recipe" is still valid. We have been working with the powder suppliers to assist them in developing new data for the Remington 209 STS.
Remington 209P should not be interchanged with loads that call for the Remington 209 STS unless so noted by the powder supplier who is providing the reloading data.
You can find more information regarding our reloading components in the ammunition section of our website located HERE .