Briggs engines have had governors since at least the 1920s Ty, as did most stationary engines at the time. For 4 stroke engines they were an essential feature rather than an advanced one. Unlike port-controlled 2 strokes which require careful port design to be able to run fast, 4 strokes will run to their valve-bounce speed unless governed. In WW2, Briggs made small cheap portable generators for the military, using the air vane governor that the side valve engines are still using now.

Probably the key question is when Briggs began making vertical crankshaft engines, with air vane governors, specifically for rotary lawnmowers. The Model 6H seems to have been their first, in 1950. Here is one made in February, 1951:
[Linked Image]

You can read a little more about its history here:
http://www.gravelyguy.com/briggs--stratton-2.html

I think it is difficult to justify claims that Victas made years later than this were the first rotary lawnmowers.