ISO vs JIS headsets. identical (i think) except the fork crown diameter. there's like a 0.2mm on the diameter difference. i had to mill a fork last time I installed a new headset on a circa 1984 TREK.
the LBS let me do it myself. He had never used the tool after 10+ years in business. the mill still had the shipping wax on it. a $25 tool rental fro a $593.95 tool. sounded fair to me!

possibly maybe that was what caused the fracture during install. sure sounds like it. probably not a VO problem (unless they are not calling out ISO vs JIS.... not sure) .
and it's some really esoteric info for the bike shop owner to keep in his mind - what with 53million different bottom bracket sizes out now.