As I stated before Yellow Coach and Cab never made 1575/1609 cabs (aka the closed cab trucks with MG holes in the roof). There were a number of suppliers that supplied the kits and these trucks were modified to mount the .50BMG through various MG mounts.
Subcontractors supplied the kits and labor to do it.
Here's some factory pics of the process:
I haven't done the math, but I stand behind my original guess of 20-25% of all CCKWs being fitted with an MG mount (all cab models).