Lots of controversy about how fast old timers threw. Virtually nothing is known about the earliest measuring techniques. Did they measure average speed (as a watch would do) or plate speed? Out of hand measurement wasn't possible until sports radar arrived a few decades ago.
Doubt many SABR types know about that 1946 newsreel which is extraordinary in that it shows the equipment used to measure Feller's speed.
Newsreel announcer calls the device a "lumiline chronograph." There are some references on the web to a lumiline screen being used to measure ballistic speed even fairly recently.
It's well known that the ball slows about 6-7 mph before reaching the plate. Feller's speed is being computed from the time it takes his pitch to pass thru the two frames resting on the batters box.
Note that Feller was gunned over a small number of pitches. For all we know, that may have been an off day for him
Any comments about his delivery which to me looks effortless?