Eventhough it is an artillery (according to serial number, notch cut, front sights, etc...) with a 4" barrel, can a stock still be attached legally??

NO, barrel length will always be the determining factor in legality

Thank you

You could just buy a new artillery barrel, put your rear sights on it, have it blued, and have yourself a nice Artillery shooter...

One went for $157 on GB a week or so ago...Interestingly, it was sold by Sarco...I have one of their artillery barrels, and they are very nice...

http://www.gunbroker.com/Auction/Vie...Item=206332493

Alternately, you could swap the 4" barrel + sight + toggle (if your toggle does not have the rear sight) for a regular 4" barrel + toggle...I'm sure there is someone here on this forum who has the parts and could perform the swap... :p