"both" is an odd statement. Both *whom*?
Here is a picture of Steve Wozniak's signature (aka "Woz"):
Image
This is from a printed-on signature on the Apple IIGS.
However, Woz will sign
just about anything you want. It doesn't cost much, and it doesn't add a lot of value.
Steve Jobs wasn't known for signing things, but I wouldn't count on a random signature being his. If it's anyone but Woz' signature, you'd have to contact the person in question to ask for verification that it is their signature. (Of course, if it is the 'plaque' that was put on the back of some pre-launch Macintosh systems, it could very well be a real signature from Jobs, and if so, the plaque itself will act as proof of authenticity.)