Copyright Has Changed:
Okay, this is what I do for a living, so relax - I'll make it easy as possible.
The Copyright Term Extension Act of 1999 extended copyright on most things. An appliance manual is considered a "work of corporate authorship." and the term of copyright was extended to 120 years after creation or 95 years after publication, whichever endpoint is earlier. (For most practical purposes, the publication date plus 95 years is the endpoint)
The exception is an "abandoned copyright," where a company publishing something is out of business, and there is no longer anyone holding that company's assets. The example here might be Lustron houses; the company is long gone, and nothing of it remains under the "corporate umbrella" of an existing company. Therefore, reproducing Lustron sales literature and manuals is okay.
But caution is needed on companies that are out of business, because often their assets are purchased by other companies that end up holding their copyrights. The example here would be Servel, a company whose brand has not been seen in the marketplace very much since 1957, but whose assets were acquired by Whirlpool, making Whirlpool the current copyright holder for Servel stuff. So, it's essential to be certain where a company's assets ended up, if they seem to be out of business.
Once you find out who owns the rights to a brand name, usually a call to the company's Media Relations department or Rights & Permissions department will result in a chance to talk with the right person about what you want to do. The response you will get varies widely - some companies couldn't care less about their 1961 owners' manuals, while some (like General Electric), don't want anything republished in any medium (including the Web), without permission. In my experience, most companies are pretty happy to be asked, and grant permission readily if they are properly credited; my experiences with GE fall into this category.
If you are planning to sell a republished manual, the company may very well want a licencing fee. Fair Use provisions of copyright law don't usually apply to republishing something in its entirety and then charging for it; you are profiting from the intellectual property of others.
Hope this helps a little; get back with me if you need more info.