I believe that Gibbs probably barreled the gun with their own barrel and their style of sights.
The stock may just be from Steyr.
I had a similar rifle which had a "B" prfix number as well. It is my understanding that this is how Gibbs identified rifles which they rebarreled, either to resell as new or even in the case of rebarreling one of their own previously sold rifles as in the case of my Farquharson.