Quoted from Robotworkshop:

I know this is an old thread but I am currently working on a Spectra IV. I was able to read the contents of my three original PROM chips. U3 and U5 matched what was on IPDB.org but my U4 is different. I saved an image of that and am going to do a diff on the images to see what is different. My hope is that perhaps the image in my machine may prove to be a good original set. I'm just about done rebuilding my MPU board and will see if it runs.