Where are the numbers on a Mossberg 151m-b?

If you mean the serial number- there is none. Rifles and shotguns (like your Mossberg) made prior to the 1968 Gun Control Act were not required to have a serial number. And yes, it is perfectly legal to own a firearm with no serial number. It is highly illegal to REMOVE a serial number, but guns that never had one are a different matter. I have 9 of the older Mossberg .22 rifles- only the one made for the US Military has a serial number.