mre12ax7 wrote:

could this thing have been involved in WWII?

Involved? You mean did it fight on the beaches at Normandy, or in the jungles of Guadalcanal? 

Brass tags are not uncommon. They were often used as part of a company's inventory control and the number on it an iternal serial number. Don't forget, typewriters were extremely expensive machines in their day, so it was normal to have an anti-theft measure of some kind in place. To answer your question you would have to figure out what "SOP" stands for. My guess would be that it's an acronym for some company or newpaper. Here's a brass tag on an Olivetti in my collection that was originally owned by a daily newspaper in Montreal: