I think your numbers are off as to how many original Miller-Meteors were bought and used in the original film,
but thats a whole other conversation.
Miller-Meteor is(was) a coach builder...who modified Cadis (and other models like Suburbans) for GM to be Hearses and Ambulances.
As far as I know their cars were produced only for those markets...
ordered privately, and not sold to the general public.
*
https://en.wikipedia.org/wiki/Cadill...ercial_Chassis
One could argue that they are built from unfinished/unsold Cadillac chassis',
and then modified and delivered
as custom built Ambulances OR Hearses, so thats what they started life as.
They are closer to Ambulances than the modified Vans we use today,
since those are produced and sold as Transport/Cargo Vans first,
and then aftermarket modified into Ambulances.
Speaking of ambulances:
https://en.wikipedia.org/wiki/Ambula...ance,_1949.jpg