I believe that's a newer version of the Willem that will, in fact, produce the necessary 25v programming voltage. But, at the moment, your problem is that the chip is not getting blank. That is a very old style EPROM, sometimes those do take longer than the newer ones to erase. According to the datasheet, it needs to be one inch away from a suitable UV light for 15 to 20 minutes.
How long was it in for?
Read it in, and see if it's "getting blanker" - most bytes should be FF, or something thereabouts.
Try programming it with all 0's, and see if it will take. You can program all 0's over anything, it does not need to be erased first - since you're just going through and programming all the bits that aren't programmed yet, writing to every cell.
-Ian