200A sounds interesting, as that was their very first model number. Your relative didn't have anything to do with Disney, did they?
Does it look like the one in the photo on that site? Sounds like a very cool artifact. You can't always tell much from the serial number -- for example, notice that they started their model numbers with '200,' just to make the company look bigger and more reputable.
There are a few HP 200As listed on eBay in the $100-$300 range. Not sure if they're actually selling for that, though.