The full article by George Plimpton is available in the Sports Illustrated archive. An excerpt is provided below.
The phenomenon the three young batters faced, and about whom only Reynolds, Stottlemyre and a few members of the Mets' front office know, is a 28-year-old, somewhat eccentric mystic named Hayden (Sidd) Finch. He may well change the course of baseball history. On St. Patrick's Day, to make sure they were not all victims of a crazy hallucination, the Mets brought in a radar gun to measure the speed of Finch's fastball. The model used was a JUGS Supergun II. It looks like a black space gun with a big snout, weighs about five pounds and is usually pointed at the pitcher from behind the catcher. A glass plate in the back of the gun shows the pitch's velocity—accurate, so the manufacturer claims, to within plus or minus 1 mph. The figure at the top of the gauge is 200 mph. The fastest projectile ever measured by the JUGS (which is named after the oldtimer's descriptive—the "jug-handled" curveball) was a Roscoe Tanner serve that registered 153 mph. The highest number that the JUGS had ever turned for a baseball was 103 mph, which it did, curiously, twice on one day, July 11, at the 1978 All-Star game when both Goose Gossage and Nolan Ryan threw the ball at that speed. On March 17, the gun was handled by Stottlemyre. He heard the pop of the ball in Reynolds's mitt and the little squeak of pain from the catcher. Then the astonishing figure 168 appeared on the glass plate. Stottlemyre remembers whistling in amazement, and then he heard Reynolds say, "Don't tell me, Mel, I don't want to know...."-30-