Brake Boosters getting scarce?

So I placed an order for the 50-3714. I got an email the next day that it was cancelled, with no reason. I call, and of course, it's not available.
I'm ordering a 50-6101, which is supposedly in stock, and Cardone's website shows this as being for 73-75 A-bodies with disk brakes.
Crossing my fingers this time....