After reviewing all of the very helpful advice here, and then contacting David Clark, I understand the problem. In all cases the connectors have been compatible (all U-174 type). Apparently some helicopter's systems are mic and phone polarity sensative, while others are not. David Clark also says that the impedance is not the problem, as the helmet's operating elements are equivilent to a DC H10-36 headset, which I'm told is still a current product, and will work in any civil helicopter.
The helmet I have was only made for a few years ending in 1985, and is so little known that it is not listed in any of their parts history, and the service rep had never heard of it. Only the David Clark tag sewn into it convinced him that it was not another company's product. It would appear that it was wired differently, and needs to be changed to be compatible and up to date. Funny that it worked in brand new Sw300's but not the much older H500 and B206! DC confirmed that the wiring diagrams offered in their parts pages of the website are the same plug pattern for all headsets which use the U-174 plug. Any different arrangements are now historical.
I was very impressed at the resorces which the David Clark staff devoted to solving my dilemma. The service was prompt and very thorough. I certainly recommend David Clark for good after sales service, even 26 years later!
Thanks to those who offered help to my inquiry, it made the difference!
Pilot DAR