What size seat does a pfister valve body 01-31XA series triple handle tub and shower use?

I can't say, Drobles, but in most cases, you remove the existing seat and then match up the seat as needed, right? An even better approach would be to contact price pfister and ask them! You can contact them at:

About Us - Company and Customer Service Contact Information | Pfister Faucets (http://www.pfisterfaucets.com/about/contact.aspx)

Let us know how you make out, OK?

Mark