The "AI" that I have in stock is round. It measures 1.355 inches by 5 inches. I use it for nedium size knife and have "not" had it rejected. I did not get it from Masecraft Supply, although I buy from them. Their sizes were just plain too much work for the very small amount of orders I receive for AI.
The shop will reject your handle material if it doesn't measure...period! Being 4 years behind has caused them to "just say no" to un-acceptable dimensions. Best to get it, cut it close to specs, siding slightly on the larger side, and send it in.
I have all but stopped taking customers handle material because it...many times...is asking me to "fudge" one way or the other. I'd rather use material that I have bought, ordered or cut to proper size...It's just plain easier.
With regards to stag (or any other natural material)...what you see is what you get. The shop does very...VERY little custom trimming on their stag. The reason they can do this (or not do this, depending on how you look at it) is the fact that the stag has been "sorted". There are bins / barrels with different diameter stag in each. They simply have to go to the correct bin to choose the handle prior to making the knife.
Hope this answer helps...even if it's not quite you wanted to hear. Buy the stuff larger and trim it. Thank God I have access to Gene Beal and his fine (but small) shop. Any problems that I have with size of materials...I pay the man for his time and get it done..."RIGHT"!!
Best, Capt. Chris