This particular one is a WAM4P only. It was written on the side behind the door, but looks like it faded away. Check out this pic ( from Flickr). the 'P' at the end is not an official requirement. Some locos, especially the ones that were converted to WAM4P specs later on, were not given this suffix on the locomotive. Some sheds chose to show the suffixes while others didn't.
Edit: My apologies for not being specific in mentioning the picture source. The picture credit goes to Abhinav Phangcho Choudhury, from Flickr.