Sure it is. Bbl length less than 16”, not intended to be fired from the shoulder ( the “brace” at the rear is not a butt stock per BATFE rules).
The “brace rules clearly indicate that although the “brace can be used incidentally as a shoulder stock, it is not design as such. It is designed to stabilize the pistol when fired one handed and clamps around the firing hand forearm....
Only problem with such “rules” is they are indeed subject to change and then the owner either has to modify or declare it as AOW or SBR and figure out how to dispose or register it appropriately.
“It’s the law, it doesn’t have to make sense” as stated by a local MO firearms attorney.
That might work as long as you don't find yourself in front of a judge and jury with eyes and common sense.