I bought this (mount) off of eBay for $40 shipped. The packaging said it was for an A391. I bought it because I have a Walmart A390 with a camo A391 youth stock. I have short muscular arms. Anyway, it didn't think about the squareback until I started to mount it and realized that it was not going to work on my model. Should work fine on a 390 without the squareback. BTW, I love that gun. I knew if it didn't fit I could get what I paid for it. I also knew that I was going to try it on my son's A300. It appears to fit very well on the A300. We haven't had a chance to take it to the range but I don't think there will be any surprises at the range. The first pic is my son's A300 with my A391 youth stock mounted on it and of course the second pic is the Speedbead mounted on his A300.
Not sure what we will use it for cause he has a Super Nova that he will use for turkey. He might try it at the range just for kicks.