I'm not an expert but ... with an intact Ne Plus Ultra logo, I'd be inclined to think 1963, or earlier.
I read somewhere, either on TradGang or another forum, that after Damon Howatt died, the older logos continued to be used on several models until the supplies were exhausted. As they ran out, the new logos took over.
Perhaps Larry Hatfield or one of the other knowledgable Howatt experts can confirm or correct that for us. I'd really like to know for sure as I have some 1960s Howatt bows too.
Mike