I appreciate everyone's help a lot on this. Sam gave me the number of a gentleman who knows his slicks very well and it seems to be pinned down to '69 or '70. Plus they also appear in a '71 catalog. I sent an email to M&H directly a long time ago and have no response yet.
Anyway, I don't think they'll do for the earlier car I had in mind. They'd still look cool, but they wouldn't be "period".
Thanks again guys!
Verne