The Mosrite-made Black Widows were very few in number, probably less than 200. While from a distance they could be mistaken for a Japanese Black Widow, there are several fundamental differences in the 2 models. The table below illustrates those differences. Also, if you are familiar with Mosrite guitars you can recognize the Mosrite hardware instantly.
Note that these are not written in stone. There have been instances of mixed features such as a 3-bolt neck and Mosrite nut or imprinted serial numbers on Japanese fretboards.
Probably the most definitive way to determine origin is by the neck joint. Japanese and Bartell instruments should all have the 3-bolt neck attachment (2 in back and 1 in front below the bridge pickup). The Mosrite models will have a 4 bolt attachment on the back.