The 1948-51 Chevrolet Truck Shop Manual reprint and the 1947-54 GMC Truck Shop Manual reprint both mention an X on the flywheel and an X on the clutch cover. I assume the flywheel/clutch was balanced as an assembly at the factory then marked thus. I highly doubt you can find 10 trucks now with the same clutch it left the factory with so I wouldn't imagine it makes much difference.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.