Found something that said MW model B is a later style National Rotary machine. Could be badged "Damascus" also. However National used that particular name for MW for a lot of years. Don't know if they called all of them Model B or changed is as the machine changed.
National used names like ELdredge B,(EB) Improved Eldredge B (IEB), Expert B model T (BT), Then they had Rotray Model A and Rotary Model B.
then there is the late model B, and then the RBR, and then the RBRW (aka streamliner), and then they get to rotary model C.
You'll see Model B down the page. If it's that model, then was probably made between 1920-1935
http://www.needlebar.org/main/national/index2.html