The threads are different. Falcon uses an interrupted thread that is almost impossible to cut without special equipment. Their threads are actually cast in. You can find a lot of bowls that fit the Viking standard threads. Most other metal pipes used it.
However, because of the continued popularity of the Falcon, custom makers are using an O-ring to seal their bowls to the stem. Because the fame is the same on both pipes, they are supposed to fit either.
I have tried two on my Vikings and they didn't work at all. They were the same on a Falcon I had. Now I must say that these two were not by Don Warren. I would expect his to fit either pipe. But understand, these bowls are not screwed on, they are plugged in like a stopper.
The bowls from a KOOLSMOKE pipe works great on a Viking stem and there are a lot of them out there. I have a Viking stem that was my first pipe (1973) but have lost the bowl. I picked up several Koolsmoke pipes and extra bowls on eBay.