This is the very popular industry standard 3029 P Spa Pressure Switch, made by Tecmark-Tridelta. The "P" Stands for Plastic, as this switch has a plastic thread. Whether to go with a plastic or stainless thread is a matter of choice, they both work great. If you prefer Stainless, the item number is # 3029.
Installation Note: When installing this switch, remember to wrap 3-4 layers of Teflon Pipe Tape around the thread, to create a good seal. We are including a FREE roll of Teflon Tape for you.