Something that probably should be mentioned is that all calls are custom made to a degree. Saw a comment by Sadler McGraw on the GON forum the other day... he stated there's no such thing as a "commercial diaphragm call machine."
In the industry, the larger companies like Primos and HS Strut contract call builders to make their calls for them. And... if companies like Woodhaven can't get all their quota with "their people," they will occasionally do the same. In fact, I have a friend who has a friend who stretches calls for Preston Pittman.
McGraw said the funny thing is that he's heard some folks claim they couldn't blow one brand of call... but be able to master another brand when it turns out that both were made by the same person.
TIFWIW
If this is true, it would not surprise me. A couple of thoughts:
First, having many different people build the same model call would probably lead to variations among calls that were intended to be the same. A true "custom" call maker - ie, one person, making fewer calls, would probably have higher repeatability of construction. Thus, "custom" calls could easily develop a reputation of being better because one call to the next would perform similarly.
Mouth calls are a very unique beast, because not only does the construction vary between companies, but more importantly to me, every person's mouth is different. I've seen guys use so much tape that it would make me vomit to put that call in my mouth. On the other hand, some people can call with no tape at all - just the little metal horse shoe.
In the end, we're talking about a pretty low level of technology here. Excluding personal preferences in shape, texture, or tone, the variations in calls is pretty small. The most important thing is getting a call that fits in YOUR mouth, and keeping it there until you can make it talk.