Just picked out a few quotes from Stephen Archer:
Stoeger X10/20 and X50 Performance
"I'm finding these BAM-manufactured guns to be accurate and consistent. My test target group sizes are now about the same as for QB78 family air rifles as I become more proficient shooting the X10, X20 and X50. BTW, the X10 and X20 are the same mechanically, differing in stock style and scope fitted when in Combo form."
And then from his http://www.archerairguns.info/search/label/Stoeger%20Airguns]Blog.
"The Stoeger X10 and the Crosman Quest 1000X are "the same" air rifle. Both are manufactured by BAM in China and carry US brands. Both are based on the B18/19 platform and have similar specifications."
Stephen Archer has written a small book on the Stoeger Airguns, called the Book of the Stoeger X10 X20 X50 version1
I suggest you keep an eye on his blog and his postings on Network54. The last book (a service manual regarding the QB78) he wrote, is being used by SAG (Shanghai Airgun Factory) the company that manufactures the QB78.