Hello, my name is ***** ***** I am pleased to assist. As you likely know, William Primrose (1904-1982) was a master of the viola and your item was made as a copy of one he would have endorsed; i.e., made in the style of those he played. "WM" is the early 20th century abbreviation for "William." It was sold by the Adrian Music Company in New York City, who imported musical instruments primarily from Germany in the 1890-1920 era. In guides, the company is spelled both Adrian Music and Adrien Music--not sure exactly which is correct, but they appear to be the same company.
The exact maker of you viola is unknown, but it's origin was likely German, made by one of scores of companies that sold musical instruments to the retail store trade in the era. Most are very well made and command from $1,000 to $2,000 pending actual condition and demand at any given time. I do wish I could tell you more, but again, as the item was not marked by a particular maker, it is near impossible to determine who made it--hope you understand.
You could try searching New York reference rooms to see if they have a list of vendors to Adrian/Adrien in the late 19th and early 20th century, but the probability is low.
If I may assist you further in any manner, PLEASE just ask.
Have a blessed weekend.
With kind regards,