Thanks for getting my name out there guys. I'd like to remind everyone of another great feature. I send free replacements for for all my printed parts. Doesn't matter if your dog eats it. It's free. You'll never need to buy another arm again.
Here's the facts. The p3s yaw and roll are both different from the p3p/a. There are lots of changes on the roll, but only a couple on the yaw. The stem of the yaw arm is 2mm longer on the horizontal and 1.5mm on the vertical. This doesn't sound like much, but the ribbon fits pretty crappy if you don't use the right arm. You can't tell by pictures.
I think ATKdrones.com is the only one to offer a metal p3s yaw arm, but you'll need to make sure before you buy. Most sellers will tell you they have the p3s arm because they want your money. Make them tell you what the difference is. As far as I know, nobody makes the p3s roll arm except for me. They sell the p3p/a arm and say it works for the standard, but it only gets the job done. It's 1.5mm longer and some screw locations don't lign up. If you care more about metal than you do strength, you can use the p3p roll arm.