Hey Jason,
According to Concord's Vietnam Armor in action, the "graffiti" on the M48 turret is actually a warning that reads: Danger, do not ride bicycles along side. "The warning was used due to the increassed usage of the 69th Armor in securing the highway system in the Central Highlands."

The photos on pages 14 and 15 in Squadron's M48A3 in Vietnam that you're referring to are also with the 69th Armor attached to the 25th Inf. Div. Presumably, that's the unknown (according to the decal sheet) unit "Hell Cat" is from.