Hi Cliff, Dano from v8Buick.com here & my first post on your forum. I don't know much about carbs tbh but have your book & probably need to learn.
As you noted above, Stage 1 carbs (auto & manual) had the POE holes below the flaps. All of the '70 350 4-sp (7040245) carbs I've seen have the holes above the flaps. The 245 does use a different air horn (7038473 for manual 7038474 for auto) than a 350 automatic (244) but I can't imagine that every 245 I've seen (maybe 10 or so - pretty rare carb) has had the air horn changed.
I had my 245 for my '70 GS 350 4-sp rebuilt by Fred Catlin (old Buick racer) and he put on an air horn w/the holes below. I have no idea what it came off of. I am looking to put this carb on a stock "appearing" build, but the internals will be far from stock (higher comp., stroked, big valve ported heads, etc.). What is the functional/performance difference between above/below & is there any reason not to or issues with changing the air horn to one that has the holes below?