Munson game caps have sold at auction for upwards of $10,000, and this fine example was sourced from batboy Felix Martinez of the 1977 Yankees championship squad. Martinez passed away in 2009, at which point the family sold the cap to collector Mark Southerland, whose notarized LOA accompanies. Manufactured by KM Pro, the size 7-3/8 cap exhibits heavy sweatband wear and features Munson's uniform #15 notated on the brim underside. Interestingly, the pre-1978 timing is accurate because '77 happens to be the very year when KM Pro ceased operations and the Yankees began using New Era caps.
A second LOA comes from expert authenticator Phil Wood, who concludes: "The Yankees certainly wore KM Pro caps for most of Thurman Munson's career. Other documented Munson caps have also been 7 3/8 (or occasionally 7 1/4) but size variance is fairly common with this particular size. Having the privilege of knowing a large number of former MLB players through my profession as a broadcaster, I checked with someone who was Munson's teammate in New York for 2 seasons, who recalled in detail that Munson wore size 3/8 because he did as well. Therefore, I'm comfortable sharing my opinion that this is an authentic New York Yankees cap as worn by Thurman Munson prior to 1978."
Batboy Felix Martinez ultimately worked as the Assignment Editor and NJ Bureau Chief for WNBC News 4 New York from 1983 through 2008.