Question about coachbuilt production and "model years"?

This came up in another thread and I thought it would be interesting to get some clarification...

As you know, most car companies these days release "new" cars to the public well ahead of their stated "model year", i.e: you can buy a 2012 car now, in the middle of 2011. What I am wondering is, did commercial chassis cars that were coachbuilt by other firms follow suit with the automakers? In other words, when the 1965 Cadillac was available to the public, would a commercial chassis car like a hearse or ambulance also be available to the public? Or did the firms like Superior, Miller-Meteor, etc. have to wait to get the chassis, then start building on it? Did the automakers make the commercial chassis available to the builder early enough that a 1965 ambulance could be introduced to the public alongside it's new model year counterpart, or were they always lagging some period of time behind?

I would think that "conversion" type cars, cars that started as a whole sedan or station wagon might lag behind the model year production since those builders retrofitted an "off the lot" car, so to speak, but I'm wondering what the case was with cars built on a commercial chassis?

In the days of the commercial chassis, professional vehicle manufacturers worked closely with Cadillac and other chassis suppliers. They learned about annual model changes well in advance (which allowed them to make any necessary styling alterations to their bodies) and received their initial chassis for each new year in the late Summer or very early Fall. This enabled them to have display cars built for the annual NFDA convention in mid-to-late October where everyone launched their new funeral cars and ambulances. Major manufacturers still enjoy a close working relationship with Cadillac and Lincoln and are supplied with special "de-contented" and up-graded platforms with which to work. IN the past, new professional cars were always launched in September or October - whenever the NFDA was held. Various manufacturers would often release new mid-year offerings in the Spring and have them on display for the Spring and Summer State Conventions but, new models were as a rule introduced in the Fall at the NFDA show. I hope that this answere your question.