Navajo Mark on a Concho Belt

I bought this belt at Sotheby’s in the mid 80s. The mark reads U.S. Navajo 4. Any ideas?

<img src=/uploads/db1846/original/1X/6fd4d8e79121f9de3b99afaf9db35c6a783d9a44.jpg">

Local governments, state governments, traders, artists & the federal government have always worked to protect the authenticity of Native American art. I believe these are numbers assigned to traders to certify that it is Navajo made. Could you show us the other side of the belt? Thanks for sharing.